THE TEMPLE PRIMERS
MODERN CHEMISTRY
Systematic
By
WILLIAM RAMSAY, D.Sc.
JOHN DALTON
mODERH
CHEmiSTRY
SYSTEI12ATN
LAlttSAY'DS?
1900* ±9 &5O BEDFORD-STREET*
All rights reserved
MODERN CHEMISTRY
SECOND PART
SYSTEMATIC CHEMISTRY
CHAPTER I
Methods of Preparing Elements — Their Physical
Properties.
Mixtures and Compounds. — In the olden days, no
distinction was drawn between a compound and a mixture.
Indeed, all " impure " substances artificially prepared were
termed " mixts." It was only after the true idea of ele-
ments had been arrived at, and indeed not until Dalton had
formulated the laws which go by his name, that the distinc-
tion was drawn. The ultimate criterion for combination is
definiteness of proportion, and this is generally connected
with uniformity in properties, or homogeneity. A sub-
stance is said to be homogeneous when no one part of it
differs from any other part in composition. But this may
be predicated of glass, or of air, which are mixtures, and not
compounds. A mixture may be homogeneous ; a com-
pound must.
Again, it is usually accepted that the separation of the
constituents of a mixture may be effected by mechanical, or
at least by physical means ; whereas the separation of the
elements from a compound require chemical treatment.
Here it is difficult to draw a sharp distinction. The
VOL. II. A
385213
2 MODERN CHEMISTRY
separation cf carbon dioxide from soda-water by the appli- .
cation of heat is similar in character to the separation of
sugar from water by evaporation of the water ; yet we
believe that a solution of carbon dioxide in water consti-
tutes a compound, while that of sugar in water is a mere
mixture of the two. It is necessary to be guided by analogy
in the former case ; and it is probable that the compound
named carbonic acid is really contained in a solution of
carbon dioxide in water, on account of the formulas and
behaviour of the carbonates.
The Atmosphere. — In the case of mixtures of gases,
the problem becomes an easier one. For in this case, each
gas retains its individual properties. The atmosphere, for
example, is believed to be a mixture of the gases
Nitrogen, . . 78.16 per cent.
Oxygen, . 20.90 „
Argon, &c., . . 0.94 „
100.00
if small amounts of water-vapour, of carbon dioxide, and
of ammonia, all of which vary considerably in amount, be
subtracted.
This can be shown by several lines of argument.
First, The density of air agrees with the mean of the
densities of its constituents, taken in the proportion in which
they occur. Thus, the density of the mixture of atmos-
pheric nitrogen and argon differs by only I part in 40,000 from
that calculated from their relative weights, and the proportion
in which they occur. This is the case with compound gases
only when the constituents are present in equal proportions
by volume, as in hydrogen chloride, HC1. The above
mixture is far from fulfilling that requirement.
Second, The constituents of air can be separated by
diffusion. Thomas Graham discovered that the rate of
escape of gases through an opening, or of passage through
THE ATMOSPHERE 3
a porous partition is inversely in the order of the square
roots of their relative densities. Now, air has been enriched
in oxygen and in argon by diffusion ; the lighter nitrogen
passes more rapidly in the proportion of
— /= : ,— : —7—,
0/14 \/i6 V20
the last two fractions referring to the rates of oxygen and
argon respectively ; the oxygen and argon, being more slowly
diffusible, are left to the last.
Third, The constituents of air may be separated by
solution in water. While oxygen is soluble at atmospheric
temperature in the proportion of about 3 volumes in 100 of
water, nitrogen is much less soluble — about 1.5 volumes ;
and argon about 4.1 volumes* Hence, on shaking air with
water, the relative volumes dissolved are :
Oxygen, 3 x 20.90 ; Nitrogen, 1.5 x 78.16 ; and
Argon, 4.1 x 0.9 4,
or in the proportion of 63 : 1 17 : 3.8. It is evident
that the relative proportion of nitrogen has considerably
decreased.
Fourth, The elements contained in air are not present in
any atomic ratio. To ascertain the relative number of
atoms of these elements it is necessary to divide the per-
centage amount of each by its atomic weight ; thus we have
Nitrogen, Z_: _ = 5.58 ; Oxygen, ^9 = 1.31 ;
Argon, 2^51 = 0.024 ;
40
and these numbers bear to each other no simple ratio.
Lastly, it is possible by distilling liquid air to separate
the more volatile nitrogen from the less volatile oxygen
and argon.
For these reasons, and other similar ones, it is concluded
that air is a mixture.
4 MODERN CHEMISTRY
The Analysis of the Atmosphere is, however,
always performed by chemical means, for the difference in
physical properties of its constituents is not sufficiently
marked to allow of their being utilised for purposes of
separation. Many common elements unite easily with
oxygen to form non-volatile compounds, when they are
heated in air. One of the most convenient for this
purpose is metallic copper. By passing a known volume
of air over copper turnings, contained in a counter-
poised tube of hard glass, and heated to redness, the
oxygen of the air is removed, for it combines with the
copper to form non- volatile black oxide of copper. The in-
crease in weight of this tube gives the weight of the oxygen in
the measured volume of air. But it is customary to analyse
air volumetrically by absorbing the oxygen from a known
volume by means of burning phosphorus, or of a solution of
potassium pyrogallate : the remainder consists of a mixture of
nitrogen, argon and its congeners. The separation of these
gases from each other is described in the next paragraph.
Reference has already been made in Part I. to the different
processes which may be used for the isolation of elements
from their compounds. But there exists a group of elements,
that of which the first member is helium, which form no com-
pounds, and which therefore are found only in a free state.
It is, therefore, convenient to begin with these.
The HELIUM Group. — These elements are all gases at
the ordinary temperature of the atmosphere, and they are
consequently all to be found in atmospheric air. They are
colourless, even in the liquid condition, and are devoid of
smell and taste. They are very sparingly soluble in water ;
for example, 100 volumes of water dissolve only 4.1 volumes
of argon at 1 5°. Their preparation consists, first, in the
separation of the other constituents of air from them, and,
second, in their separation from each other.
Air, which is a mixture, and not a compound, of nitro-
gen, oxygen, carbon dioxide, ammonia, water-vapour, and
the gases of the helium group, is a supporter of combustion,
THE HELIUM GROUP 5
owing to the combination of the oxygen which it contains
with most other elements. Now, when air passed through
a tube full of a mixture of caustic soda and lime, to remove
carbon dioxide, and then through a U-tube containing sul-
phuric acid, to deprive it of water-vapour and ammonia, is led
over red-hot copper, or over some other red-hot metal which
unites with oxygen, the oxygen is retained, and nitrogen with
members of the helium group alone passes on. The nitrogen
can be removed in one of two ways. The first plan is due
to Cavendish, who attempted to prove that atmospheric nitro-
gen was a homogeneous substance. He mixed atmospheric
nitrogen with oxygen, and passed electric sparks through
the mixture, having a little caustic soda present in the
tube. Under the influence of the sparks, the nitrogen and
oxygen combine, giving nitride peroxide, NO9 ; this com-
pound is absorbed by the soda, with formation of sodium
nitrate and nitrite, NaNO3 and NaNO2. Cavendish
obtained a residue of not more than one-hundred-and-
twentieth of the nitrogen ; and he concluded that if
atmospheric nitrogen was not homogeneous, it contained only
a trace of another gas. The second plan is to pass the
atmospheric nitrogen over red-hot magnesium, or, better, over
a mixture of magnesium powder and lime, which gives
calcium ; the magnesium or the calcium unites with the
nitrogen, and the inert gases pass on.
To separate these gases from each other, they are
compressed into a bulb, cooled to -185° by being immersed
in liquid air. The argon, krypton, and xenon condense to
a liquid, in which the neon and helium are dissolved. On
removing the bulb from the liquid air, its temperature rises,
and the helium and neon escape first, mixed with a large
amount of argon. Argon distils next, and krypton and xenon
remain till the last. By frequently repeating this process
of " fractional distillation," the argon, krypton, and xenon
can be separated from each other, and from the helium and
neon which still remain mixed with each other, for both
are gases at the temperature of boiling air.
6 MODERN CHEMISTRY
To separate helium from neon, recourse must be had to
liquid hydrogen. To liquefy hydrogen, the process is in
principle the same as that for liquefying air, described on
p. 26. The hydrogen, compressed by a pressure of 200
atmospheres, is cooled to —205° by passing through a coil of
copper pipe, immersed in liquid air boiling under low
pressure. On expanding, its temperature is still further
lowered, and the still colder gas, in passing upwards, cools
the tubes through which the compressed gas is passing.
The hydrogen finally issues in the liquid state, as a colour-
less, mobile liquid, of the approximate temperature —240°.
By its aid, if a mixture of neon and helium is cooled to
—240°, the former freezes, while the latter remains
gaseous. The gaseous helium can be removed with the
pump ; and the neon, after it has been warmed, may also be
pumped off in a pure state.
Helium can also be prepared by heating certain specimens
of pitchblende or uraninite, a mineral consisting chiefly of
oxide of uranium. The gas, which appears to exist in some
sort of combination with the uranium oxide, escapes ; it
contains a trace of argon. All these gases give very striking
spectra, and that of helium was observed during the solar
eclipse of 1 868 in the chromosphere, or coloured atmosphere,
of the sun. Although at that time it had not been dis-
covered on the earth, the name " helium " was given to
the bright yellow line, which is the most characteristic of
its spectrum.
As regards the relative amount of these gases contained
in air, 100 volumes of air contain 0.937 volume of the
mixture. By far the largest portion of this mixture is
argon ; probably the volume of all the others taken together
does not exceed one-four-hundredth part of that of the
argon. Indeed, it may be said with truth that there is less
xenon in air than there is gold in sea-water.
Methods of Separating Elements from their
Compounds* — The methods of preparation of the remain-
ing elements depend on considerations of the cost of the
SEPARATION PROCESSES 7
compound from which the element is to be prepared, and on
the ease of preparation. In the case of those elements which
are required on a commercial scale, like iron, for example,
the process of manufacture is regulated chiefly by the cost
of the ore, and of the operations necessary to produce the
metal in a state of purity sufficient for commercial purposes.
But if perfectly pure iron is required for scientific purposes —
for example, in order to determine its electrical properties —
then the question of cost does not come into consideration,
and processes are adopted which are necessarily very costly.
In the description which follows, however, we shall give
only the ordinary methods of preparation.
Again, the process chosen depends greatly on the physical
and chemical properties of the element which it is desired to
isolate. Some elements are volatile, and are more or less
easily separated by distillation from the material from which
they are produced ; some elements are attacked by water,
while others resist attack ; some fuse at comparatively low
temperatures, and can thus be separated, while others are
producible in a compact state only at the enormously high
temperature of the electric arc. It is necessary, therefore, to
know the properties of the element required before deciding
on a process for its isolation. The preparation of the
remaining elements will therefore be considered from this
point of view.
( i ) Separation of the element by means of an
electric current.
(a) From a fused salt. — One condition is that the
salt shall fuse at a convenient temperature — that is, at or
below a red heat. Another is that, in the case of metals
which are commercially used, the salts must be cheaply
obtainable, and the metals easily separated from the salts.
It is interesting to note that this process led, in the hands
of Sir Humphry Davy, to the discovery of the metals
of the alkalies, potassium and sodium ; he first prepared
them by passing a current from a battery of high voltage
8 MODERN CHEMISTRY
through the hydroxide, melted on a piece of platinum foil.
The metal was visible only for an instant ; for it floated
up from the electrode of platinum wire, and burst into
flame as soon as it came into the air.
As a rule, however, the chlorides are the most con-
venient salts for electrolysis. From the known fact that
the melting-point of a compound is lowered by the presence
of an "impurity," it is often found advantageous to electro-
lyse a mixture of chlorides rather than a pure chloride ;
in this case one of the elements is liberated in preference
to the other. As the anode has to withstand the action
of chlorine, it is always made of carbon, which does not
unite with chlorine directly ; the kathode may be of iron,
a metal which has no tendency to form alloys with those
which are prepared in this way, at least at the temperatures
required. The kathode may be the iron pot in which the
chloride is kept fused.
The elements which are prepared in this way are :
lithium, sodium, potassium, rubidium, caesium, beryllium,
magnesium, calcium, strontium, and barium. The first five
are easily fusible white soft metals, which take fire when
heated in air, and must therefore be kept in an atmosphere
free from oxygen ; they also attack water, liberating hydrogen,
with formation of the hydroxide MOH. Their density
is so low that they float on their fused chlorides ; they
must, therefore, be liberated in the interior of a bell-shaped
iron electrode or of a fireclay receptacle, down which an
iron kathode passes. Beryllium and magnesium are better
prepared from a mixture of their chlorides with potassium
chloride ; the latter melts and collects at the bottom of the
pot, which, in this case, may be the kathode. They are
hard white metals, magnesium melting at about 750°, and
beryllium about 1 200°. They, too, take fire when heated
in air, and burn with a brilliant flame ; indeed, the chief
use of magnesium is for signalling purposes. The metal is
drawn, while hot, into wire, which is then rolled into
ribbon; this ribbon burns with an exceedingly bright flame,
METALS OF THE ALKALIES 9
producing the oxide MgO. Calcium, strontium, and
barium are also white metals ; they have been produced by
electrolysis of their cyanides, M(CN)2, compounds which
fuse at a lower temperature than the chlorides. They are
very readily attacked by water, yielding the hydroxides
M(OH)0. The only two of these metals which find
commercial use are sodium and magnesium.
Aluminium, which is also manufactured on a large scale,
is produced from its ore, bauxite, from which pure alumina,
the oxide, is first prepared. The alumina is dissolved in
fused cryolite, a fluoride of aluminium and sodium of the
formula NagAlF6, deposits of which occur in Greenland.
The aluminium sinks to the bottom of the crucible, and
when a sufficient quantity accumulates it is tapped out.
The "flux," as the cryolite is termed, is again melted,
and a further quantity of alumina is dissolved in it. The
metal is fairly hard, white, susceptible of a high polish,
ductile and malleable. It is also very light (about two and
a half times as heavy as water), and not easily oxidised in air
at the ordinary temperature, nor is it attacked by water.
(b) From a dissolved salt. — Gallium, a tin- white,
hard metal, very rare, contained in some zinc ores, is
deposited from a solution of its hydroxide in caustic
potash. Copper prepared, as will be seen below, in a
crude state by displacement, is purified by electrolysis.
It is of the utmost importance to employ pure copper for
the conduction of electric currents ; for although copper
is one of the best conductors, its resistance is enormously
increased by the presence of a very small trace of impurity.
To purify it, large rectangular blocks of crude copper are
suspended close to thin sheets of pure copper in an acid
bath of copper sulphate, CuSO4.Aq. The heavy block
is made the anode and the thin sheet the cathode; the
sulphation, SO4, in discharging at the anode, dissolves
copper from the thick block as sulphate; while the cuprion,
+ +
Cu, in yielding up its charge at the kathode, deposits on
TO MODERN CHEMISTRY
the latter and increases its thickness. The impurities,
arsenic, antimony, and iron, remain in solution, and a
sludge is deposited containing silver and gold, besides
traces of many other elements. Copper is a very malleable,
ductile red metal, melting at 1330°.
Objects of iron are often " nickel-plated," or covered
with a thin film of nickel, a white, hard metal which pre-
serves its lustre in air, for it is not easily oxidisable. This is
done by making the object to be coated with nickel the kathode
and a bar of nickel the anode ; the liquid is a solution
of oxalate of nickel and potassium. Iron objects are first
coated with copper before nickelling. Silver and gold are
best deposited from their double cyanides with potassium ;
these salts are used because the deposit is harder and more
uniform than if a halide be used. In thus coating objects,
it is of importance that the current density, i.e. the ratio
of the current to the area of the surface of the object to be
coated, should be considered ; if this be too high, the metal
will be deposited in a loose, flocculent condition.
As an illustration of the changes which take place during
such electrolysis, the deposition of silver may be chosen.
The compound employed is, as stated, the double cyanide
(see p. 187) ; its formula is KAg(CN)2, and the ions
+
are K and Ag(CN)2. There are, however, at the same
+
time a few ions of Ag and CN. From the last, metallic
silver is deposited on the kathode ; and as soon as its
amount is reduced, a fresh quantity is formed by the
decomposition of the complex ion, Ag(CN)9. The
formation and deposition of the silver ion goes on con-
tinuously until all the silver required has been deposited.
Similar changes take place during the electro-deposition of
nickel and of gold.
Modern electrolytic processes for obtaining chlorine and
caustic soda (NaOH) from salt result in the liberation of
enormous quantities of hydrogen. The salt, dissolved in
ELECTRO-DEPOSITION 11
water, is placed in a tank divided into two compartments
by a porous diaphragm ; the anode, which consists of
carbon rods, dips into one, and the kathode, which may
be formed of copper plates, in the other. The ions, of
+
course, are Na.Aq, and Cl.Aq. The chlorine is liber-
ated at the anode, and the sodium at the kathode. But as
soon as the sodion is discharged, it reacts with the water,
forming caustic soda, thus : 2Na + 2HOH = aNaOH + H2.
Hence the production of hydrogen. Bromine and iodine
may be liberated in the same way as chlorine, the bromide
or iodide of sodium or potassium being substituted for the
chloride. As fluorine at once acts on water, liberating
• oxygen in the form of ozone, O3, it cannot be produced
from an aqueous solution of a fluoride ; but it has been found
that liquid hydrogen fluoride has ionising power, so that on
passing a current between poles of platinum-iridium (an
alloy of metals which is less attacked by fluorine than any
other conductor) through a solution of hydrogen-potas-
sium fluoride, HKF, in pure liquid hydrogen fluoride,
H^F^, at -30°, fluorine is evolved from the anode
as a pale yellow gas with a strong characteristic smell,
somewhat resembling that of the other halogens, chlorine,
bromine, and iodine ; while hydrogen is evolved at the
kathode, having been produced by the action of the potas-
sium on the hydrogen fluoride. Fluorine boils at —195°,
chlorine at -35°, bromine at 59°, and iodine, which is a
solid at atmospheric temperature, melts at 114° and boils
at 184°. The colours of these elements also show a
gradation. Chlorine is greenish-yellow; bromine, red both
as gas and liquid ; iodine is a blue-black solid and a violet
gas. These three elements are somewhat soluble in water,
and more so in a solution of their soluble salts. It has
recently been found that another ionising agent than water
may be used. Lithium chloride is soluble in pyridine, a
compound of the formula C5H5N, and may be electro-
deposited on a platinum kathode from such a solution.
12 MODERN CHEMISTRY
The metal is not attacked by pyridine ; the chlorine,
however, is rapidly absorbed.
(2) Separation of an element from a compound
"by rise of temperature.
This method is applied in practice only to the prepara-
tion of oxygen, and of chlorine, bromine, and iodine ; but
many other elements may be thus made, where the com-
pound heated does not tend to re-form on cooling. These
cases will be considered first.
Ordinary coal-gas consists chiefly of methane, CH4,
ethylene, C2H4, carbon monoxide, CO, and hydrogen,
the last amounting to nearly 50 per cent, of the volume
of the gas. This hydrogen owes its origin, at least in
part, to the decomposition of its compounds with carbon,
by their coming into contact with the red-hot walls of the
retort in which the coal is distilled. Carbon deposits in a
dense black mass on the iron, and is removed from time to
time with a chisel. Hydrogen escapes and mixes with the
coal-gas. This form of carbon is used for the pencils for
arc-lights, and for the anodes of Bunsen's and other forms of
cells, and also for anodes in electro-chemical processes.
The compounds of hydrogen with nitrogen (ammonia,
NH3), sulphur, selenium, and tellurium (sulphuretted,
seleniuretted, or telluretted hydrogen, H9S, H2Se, H0Te),
all of which are gases at the ordinary temperature, are de-
composed if passed through a red-hot tube, giving hydrogen,
which escapes along with nitrogen if ammonia be heated ;
or a deposit of the sulphur, &c., in the cold part of the tube
if one of the other gases mentioned be employed.
The oxides of the metals ruthenium, rhodium, palladium,
silver, osmium, iridium, platinum, gold, and mercury are
decomposed at a red heat ; and the chlorides, bromides,
iodides, and sulphides are also decomposed, except those
of silver and mercury.
But none of these methods are practical plans of prepar-
ing the elements. On the other hand, as already stated,
DECOMPOSITION BY HEATING 13
this method is generally used for the production of oxygen.
This gas, although it had probably been obtained in an
impure state by the older experimenters, was first pro-
duced in approximate purity by Priestley and simul-
taneously by Scheele in 1774. Priestley produced it by
heating mercuric oxide, HgO, which decomposes thus :
2HgO = 2Hg + O2. And Lavoisier showed that it was
possible to produce mercuric oxide by heating mercury to
its boiling-point in a confined portion of air, and by sepa-
rating and weighing the oxide, and subsequently heating it
till it decomposed again, he proved that the oxygen had
really been extracted from the air.
Certain oxides are not wholly decomposed into oxygen
and element when heated, but leave an oxide containing
less oxygen than that originally heated. Among these is
black manganese dioxide, a mineral named pyrolusite ;
3MnO2 = Mn3O4 + O0. Lead dioxide undergoes a similar
change: 2PbO9= zPbO + O9. The most important ap-
plication of this method, however, is the commercial plan
of producing oxygen carried out in the " Brin Company's "
works. In their process, barium oxide, BaO, is heated
in iron tubes under pressure, air being pumped in. The
barium oxide absorbs the oxygen of the air, the nitrogen
being allowed to escape. After the operation has gone on
for about five minutes, a considerable amount of oxygen is
absorbed, barium dioxide, BaO2, being formed. The
stopcocks of the pipes leading to the pump are then
reversed, so that gas is exhausted from the hot iron
tubes. When the pressure is reduced, the barium dioxide
loses oxygen, and again returns to the state of monoxide :
2BaO2 = 2 BaO + O2. The pumping is continued for about
five minutes, and the valves are again reversed. The pro-
cess is thus a continuous one ; the oxygen is not pure, for
it contains about 7 per cent, of nitrogen ; but for medical
use in cases of pneumonia, and for the oxy-hydrogen blow-
pipe, its purity is sufficient.
This method of preparing oxygen is an instance of what
I4 MODERN CHEMISTRY
is termed " mass-action." The temperature is kept con-
stant, but the pressure is raised when it is desired to cause
the oxide to absorb oxygen, and lowered when it is neces-
sary to remove the oxygen. When pressure is raised, the
number of molecules of oxygen in unit volume of the space
ifor the mass) is increased, and hence the number in contact
with the absorbing medium, the barium oxide. Combina-
tion, therefore, takes place between the two. On reducing
pressure, the number per unit volume is reduced, and the
compound decomposes. The phenomenon is analogous
with the behaviour of a vapour when it is compressed ;
after a certain pressure has been reached — the vapour
pressure — the vapour condenses to a liquid, and if more
vapour be compressed into the same space, the pressure
does not rise further, but more vapour is condensed : this
is analogous to the formation of more BaO2. On pumping
out vapour, the pressure does not fall, but the liquid
evaporates : this is the analogue of the decomposition of
the BaO2 into BaO. The law of mass-action is very
generally applicable.
Certain oxides, for instance, pentoxide of iodine, I2O5,
and of nitrogen, N0O5, decompose when heated. These
oxides form combinations with the oxides of many other ele-
ments, such as sodium or potassium oxide, e.g. Na2O.I2O5
or NaIO3, K2O.N2O5 or KNO3 ; a similar compound is
potassium chlorate, KClOg or K^O-C^O^ although the
simple oxide of chlorine is unknown. Now, potassium
and sodium oxides are not decomposed by heat, and when
these salts are heated oxygen is evolved from the pentoxide
of chlorine or iodine. These elements, however, do not
escape, but replace the oxygen combined with the sodium
or potassium, forming chloride of the metal, thus :
K2O.C12O5 = K2O+ C12 +50, and K2O + CJ2=2KC1
+ O, or, summing up both changes in one equation, 2KC1O3
= 2KC1 + 3O2. Nitrate of potassium, on the other hand,
loses only one atom of oxygen, leaving nitrite:
DECOMPOSITION BY HEATING 15
Oxygen is a colourless gas, without smell or taste ; it
can be liquefied, at a high pressure and a* low temperature,
to a pale blue liquid boiling at —182°. Most elements
unite directly with it, often with such a rise of temperature
that incandescence is produced ; in such a case the pheno-
menon is termed "combustion/' In many instances, for
example when iron rusts, the oxidation is not attended by
any measurable rise of temperature, although; in all cases
heat is evolved, but in some cases extremely slowly.
Chlorine, bromine, and iodine are generally prepared by
heating together a chloride, bromide, or iodide with man-
ganese dioxide an^ sulphuric acid diluted with water.
Here the first change is the formation of the halogen
hydride, HC1, HBr, or HI. The hydride, however, is
+ -
ionised in water, and the HCl.Aq., for example, at once
reacts with the MnO9, forming non-ionised water and
MnCl4. Aq, thus : MnO2 + 4HC1. Aq. - M n C 14. Aq. +
2H>7O. Tetrad manganese, however, appears not to be able
to co-exist with chlorine in solution ; hence the manganese
loses an electron and becomes Mn, the lost charge neutralis-
ing one of the charged chlorine ions, which escapes in an
electrically neutral state. Even then,, however, the Mn,
though capable of existence at low temperature, still loses
a charge, and a second chlorine atom is liberated in a non-
ionised state. Hence the whole change is: Mn Cl4.Aq.
+ + -
= MnCl2.Aq. + C12. Summing all these changes in one
equation, we have: MnO2 + zNaCl.Aq. + 2H2SO4.Aq.
- MnSO4. Aq. + Na2SO4. Aq. + 2H2O + C12 ; or, if hydro-
chloric acid alone be warmed with manganese dioxide,
MnO2 + 4HC1. Aq. « MnCl.,. Aq. + 2H2O + C12.
( 3 ) Separation of an element from a compound by
displacement. — This is by far the most general method
16 MODERN CHEMISTRY
of preparing elements. The elements commonly used as
displacing agents are : —
(a) Hydrogen at a red heat. — The oxide or chloride
is placed in a tube of hard glass, heated to 600° or 700°
in a tube-furnace, and a stream of dry hydrogen is passed
through the tube. Water or hydrogen chloride is formed,
and is carried on by the current of hydrogen, and the
element i-s left. Indium, thallium, germanium, tin, lead,
antimony, and bismuth are left in fused globules, solidifying
to white lustrous metallic beads ; arsenic gasifies and con-
denses in the unheated part of the tube as a grey deposit ;
tellurium, which is also volatile, condenses as a lustrous
metallic solid ; while iron, cobalt, nickel, copper, and silver
do not fuse at that temperature. The first three remain as
grey powders, the copper as a red powder, and the silver
in a white spongy condition. These metals can be fused
by heating them in a crucible to a sufficiently high tem-
perature ; it is well to use a " flux," or substance to make
them flow, such as sodium carbonate or borax ; the flux
fuses, and dissolves any film of oxide off the surface of the
metallic beads, and they then join up to form a single mass
of molten metal.
(1} Displacement by means of sodium at a red
heat. — The chlorides of beryllium, magnesium, calcium,
strontium, barium, aluminium, scandium, yttrium, lantha-
num, ytterbium, cerium, thorium, vanadium, niobium, and
tantalum are all reduced when added to sodium kept melted
in an iron crucible. For boron, silicon, and titanium the
double fluoride is more convenient, for the chlorides are
volatile liquids. The process for manufacturing magne-
sium, which is carried out on a large scale, may be more
minutely described as an example. The double chloride
of magnesium and potassium, MgCl2.KCl, carefully dried,
is mixed with sodium in proportion to unite with the
chlorine of the MgCl2, the sodium being in small lumps.
The iron crucible containing the mixture is heated ; a
violent reaction takes place, and magnesium is liberated :
DISPLACEMENT 17
MgCl.2. KC1 + 2Na - Mg + zNaCl + KC1. As magnesium
is volatile, and can be distilled, it is purified by this
operation. The contents of the crucible are treated with
water ; the potassium and sodium chlorides dissolve, and
the globules of magnesium are collected, dried, and placed
in a crucible, through the bottom of which a tube is fixed
reaching nearly to the lid, and projecting some distance
below the bottom. This crucible is placed in a furnace,
and on raising the temperature, the magnesium volatilises
up, passes down the tube, and the vapour condenses in the
cooler part of the tube which projects below the furnace.
This particular method of distillation is called destillatio per
descensum. The other elements mentioned are too little
volatile to admit of purification by this means. In their
case, the cooled mass is treated with alcohol in order to
remove the excess of sodium, and then with water to
dissolve the resulting salt ; the element is left in the state
of powder.
(c) Displacement by means of magnesium at a red
heat — This process is sometimes used to prepare the
element from its oxide. A mixture is made of magnesium
filings with the oxide of the element, and it is heated in an
iron crucible. The resulting mass is then treated with
hydrochloric acid to remove the oxide of magnesium, which
is thus converted into the soluble chloride. It is, of course,
essential that the liberated element shall not be attacked by
hydrochloric acid. The process works for the preparation
of boron, silicon, and titanium.
(</) Displacement by heating the oxide with car-
bon.— This process is of the most general application. If
the element is volatile, it is distilled from an iron or fire-
clay retort ; in this way sodium, potassium, rubidium,
arsenic, zinc, and cadmium are prepared. If non-volatile
at a red heat, a mixture of the oxide with charcoal is
heated to bright redness in a clay crucible. On a manu-
facturing scale, coal or coke is substituted for the charcoal.
The process is applicable to the production of indium,
VOL. II. B
i8 MODERN CHEMISTRY
thallium, germanium, tin, lead, manganese, iron, cobalt,
nickel, and copper. To exemplify this method, four
instances will be described — the preparation of phosphorus,
sodium, zinc, and iron.
Phosphorus. — The commonest natural compounds of
phosphorus are phosphorite or calcium phosphate,
Ca3(PO4)2, and gibbsite or aluminium phosphate, A1PO4.
It is accordingly convenient and economical to prepare
phosphorus from one of them. The process depends on
the displacing action of carbon on the oxide at a high
temperature. There are two methods of effecting this.
The first is : the phosphorite is mixed with dilute sulphuric
acid ; the hydrogen of the sulphuric acid replaces the cal-
cium of the calcium phosphate: Ca3(PO4)9 + 3H9SO4.Aq
= 3CaSO4+ 2H3PO4.Aq. Coke or charcoal is impreg-
nated with the phosphoric acid and heated to redness, when
the phosphoric acid loses water : HgPO4 = HPOg + H2O.
The mixture of metaphosphoric acid, HPO3, with carbon
is charged into retorts of Stourbridge clay, the mouths of
which are attached to a vertical copper tube, the lower
end of which dips under water. On raising the retorts to
a white heat, phosphorus distils over and condenses in the
water. The final equation is : 4HPO3+ I2C = 2H2 + P4
+ I2CO. By the second method, the calcium and alumi-
nium phosphates are mixed with silica and carbon, and
distilled from an electric furnace heated to whiteness by
an arc in its interior.
Sodium. — A mixture is made of "spongy iron" (see
p. 19) and pitch. This mixture is heated to redness in
order to decompose the pitch, which consists of compounds
of carbon and hydrogen. These compounds are decom-
posed, and a part of the carbon is left mixed with the
spongy iron, while the hydrogen escapes in combination
with the rest of the carbon. To this mixture, placed in
an iron crucible, caustic soda is added ; the lid of the
crucible, which is furnished with a curved tube sloping
downwards to a condenser, is fixed in place, and the
ZINC AND IRON 19
crucible is heated in a furnace to bright redness. The
carbon removes oxygen both from the hydrogen and the
sodium, and sodium and hydrogen pass over into the
condenser along with carbon monoxide, the sodium alone
condensing, for the others are gaseous and escape. The
equation is: 2NaOH + zC = 2CO + H2+ 2Na. The con-
denser consists of a flat hollow copper vessel ; the sodium
is raked out as it accumulates.
Zinc. — The chief ore of zinc is the sulphide. To
convert it into the oxide, it is roasted on a flat hearth in a
current of air : 2ZnS + $O2= 2ZnO + 2SO2. The oxide
is mixed with small coal (slack) and placed in cylindrical
retorts of fireclay. These retorts have pipes of rolled
sheet-iron luted to the open ends with fireclay ; they are
packed into a furnace in tiers, and the temperature is raised
to bright redness. The coal distils first, giving off coal-
gas, which expels air from the retorts. When the tem-
perature exceeds 1000°, the zinc distils and condenses in
the iron pipes. It happens that almost all zinc ores
contain cadmium sulphide, which, like zinc sulphide, is
converted into oxide by roasting ; and on distillation, the
cadmium, which is the more volatile metal, distils over
first and condenses in the outer portion of the tubes.
These are untwisted and the metal removed with a chisel.
Iron. — The chief ores of iron are the carbonate and
the oxide. The former is practically always mixed with
clay (clayband) or with coal (blackband), and generally
contains sulphur and phosphorus in the form of calcium
sulphate, CaSO4, and calcium phosphate, Ca3(PO4)9.
The sulphur is sometimes present in the form of iron
pyrites, FeS2. The ore is roasted to expel carbon
dioxide, thus: 4FeCO3 + O2 = 2Fe2O3 + 4CO2. If it
were then in its impure state smelted with coal, the
iron would not flow, but would remain mixed with the
clay. However, this process, if the ore is pure and charcoal
is used as fuel, yields a mass of iron sponge, which can be
heated and welded by hammering into a coherent mass.
20 MODERN CHEMISTRY
The process is still used by Africans, and was at one time
universal. On the large scale, however, it is necessary to
add lime in order to form a flux with the clay. Clay con-
sists of a compound of silica, SiO0, and alumina, A12O3,
and with lime it melts to a glassy slag. Alternate layers
of coal, lime, and the roasted ore are fed in at the top of a
blast-furnace, a tall conical erection of firebrick, strength-
ened by being bound with iron hoops ; at the bottom there
is a "crucible," or receptacle for the molten iron, which
can be discharged when required by forcing a hole in its
side with an iron bar. There are also holes which admit
water-jacketed tubes or "tuyeres," which convey a blast
of air heated to about 600° to increase the temperature of
combustion of the coal. Here the reduction takes place
in the upper part of the furnace, owing to the carbon
monoxide formed by the combustion of the coal in the
lower part of the heated mass ; it ac:s on the oxide of iron
thus: Fe2O3+ 3CO = 2Fe + 3CO2. As the iron passes
down the furnace it melts, and is met by the fused slag ;
it then coheres and runs into the crucible, whence it is
drawn off from time to time.
Carbon unites with molten iron, forming a carbide ; hence
the product of the blast-furnace is not pure iron, but a
mixture of iron with its carbide, and also with its sulphide
and phosphide, if the ore has contained sulphates or phos-
phates. When such impure iron is brought in contact
with oxygen in a molten or semi-molten condition, the
carbon, sulphur, and phosphorus are oxidised mostly before
the iron. If lime be present, sulphate and phosphate of
calcium are formed. The modern process of removing
these impurities is to pour the molten metal into a pear-
shaped iron vessel lined with bricks made of magnesia ;
while it is molten, air is blown through the metal, and the
carbon burns to carbon dioxide ; the sulphur and phosphorus
are likewise oxidised and combine with lime, a layer of
which floats on the surface of the molten metal. When
these impurities have thus been removed in the " Bessemer
DISPLACEMENT BY OXYGEN 21
converter," the metal is poured into a mould. Steel is
a mixture of iron with a trace of its carbide, and it is
produced by mixing with the blown iron, before it is
poured, a quantity of iron containing carbon and manganese
(a metal which confers valuable properties on iron). The
quantity of carbon in steel may vary between O.6 and 1.5
per cent. ; with the content of carbon varies also the quality
of the steel ; that with a small proportion is soft, with
a high proportion hard.
(e) Displacement by means of Oxygen. — Oxygen
is used in Deacon's process to liberate chlorine from
hydrogen chloride. The latter gas, mixed with air, is
passed through a chamber kept between the limits of
temperature 375°-4OO°, containing bricks soaked with
cupric chloride, CuCl0. At this temperature the cupric
chloride decomposes into cuprous chloride, CuCl, and
free chlorine, but the cuprous chloride is reconverted into
cupric chloride at the expense of the chlorine produced
by the interaction of the hydrogen chloride and the air,
thus : 4HC1 + O2 = 2H2O + 2CI2. The cupric chloride is
again decomposed. This kind of action, where a limited
quantity of a substance, itself not permanently changed,
causes an apparently unlimited change in other reacting
bodies, is termed " surface action," for its rate is dependent
on the extent of the surface of the agent ; and the name
"catalysis" is sometimes given to such an action. The
action would take place independently of the catalytic
agent, but at a very slow rate ; the presence of the catalyser
has the effect of greatly increasing the rate at which the
change takes place. The chlorine thus prepared is not
pure, but mixed with the nitrogen and argon of the air,
but it serves for some purposes. The rate of such action
of oxygen in displacing bromine or iodine from their
compounds with hydrogen is much greater, and at a high
temperature the elements could be formed thus, but they
are not usually produced in this way.
The preparation of nitrogen may be also regarded as a
22 MODERN CHEMISTRY
displacement by means of oxygen. Ammonia burns in
oxygen, thus: 3NH3+ $O2= 3H2O + N2, but at the
same time some of the nitrogen unites with the oxygen
and forms NO9, nitric peroxide : this gas interacts with the
ammonia, forming ammonium nitrate and nitrite, NH4NO3
and NH4NO9. If, however, the oxygen be not free, but
in combination with an easily reduced metal, such as copper,
it will combine with the hydrogen of the ammonia at a red
heat, setting free the nitrogen. Another method involves
the mutual displacement of nitrogen from its oxide by
means of hydrogen, and from its hydride, ammonia, by
oxygen: 2NH3 + N2O3= 3H2O + 2N2. This method is,
however, usually represented by the equation NH4NO0 =
2H2O + N2; for ammonium nitrite, NH4NO2, may be
regarded as a compound of N2O3 with 2NH3 and H2O.
To obtain nitrogen by this method, since ammonium nitrite
is not easily obtained, a solution of ammonium chloride may
be warmed with one of sodium nitrite. The equation is
then : NaNO2. Aq + NH4C1. Aq = 2H2O + N2 + NaCl. Aq.
Another convenient method is to warm together solu-
tions of sodium hypobromite and ammonium chloride ; the
former loses oxygen readily, which combines with the
hydrogen of the ammonia according to the equation :
3NaOBr.Aq. + 2NH4Cl.Aq. = sNaBr.Aq. + 3H0O +
2HCl.Aq. + N2.
Although sulphur, selenium, and tellurium burn in oxy-
gen, still they may be displaced from their hydrides, H2S,
H2Se, and H0Te, by means of oxygen at a red heat,
provided the oxygen is present only in sufficient quantity to
combine with the hydrogen, thus : 2H2S -f O2=2H2O +
S9. Aqueous solutions of these compounds, too, are
decomposed on standing, in contact with air, owing to
similar displacement. Oxygen may displace mercury from
its sulphide, cinnabar, HgS, which is the common ore of
mercury ; here the sulphide is roasted in air, when the
sulphur combines with the oxygen to form sulphur dioxide,
a gas at ordinary temperature ; and mercury is liberated.
DISPLACEMENT OF ELEMENTS 25
also in the gaseous form, but condensing at temperatures
below 358°.
(/) Displacement by use of Fluorine, Chlorine, and
Bromine. — Fluorine, chlorine, and bromine may also be
employed as displacing agents for nitrogen and oxygen.
A current of fluorine led through water displaces the
oxygen, forming hydrogen fluoride ; but the oxygen is in
an allotropic state (see Part i. ), called "ozone." Again,
if a stream of chlorine is passed through, or if bromine-
water be added to, a solution of ammonia, the hydrogen
and chlorine combine, while the nitrogen is set free :
2NH3.Aq + 3C12 = 6HC1 + N9; but as ammonia com-
bines with hydrogen chloride, the reaction 6NH3 + 6HC1
= 6NH4C1 occurs simultaneously; the complete equation
is the sum of these two : 8NH3. Aq + 3d., = 6NH4C1. Aq
+ N2.
Chlorine, added to a solution of bromide or iodide of a
metal, displaces the bromine or iodine ; here the non-
ionised chlorine becomes ionised at the expense of the
charge on the ionised bromine or iodine, while the latter
+ - + -
lose their charges, thus : 2KBr. Aq + C12. Aq. = 2KCl.Aq
+ Br2. Aq. Similarly, bromine displaces iodine from a
soluble iodide. But iodine displaces chlorine from the
nearly insoluble silver chloride. Here, the iodine is still
less soluble than the chloride ; and as chloride dissolves,
the less soluble and therefore non-ionised iodide is formed.
(g) Many metals are able to displace others. Thus,
iron placed in a solution of a copper salt displaces the
copper ; copper displaces silver ; silver, gold. In all
these cases the action is doubtless an electrical one, and
dependent on the replacement of a metal of lower by one of
higher electric potential ; that of higher potential becomes
ionised, while that of lower assumes the metallic state,
thus: CuCl2.Aq + Fe = FeCl2.Aq + Cu; 2AgNO3.Aq +
Cu - Cu(NO3)2. Aq + 2 Ag.
24 MODERN CHEMISTRY
(h} There are some plans of obtaining elements which,
though they can be referred to one or other of the three
general methods exemplified already, are, on account of
their complexity, better treated separately. Among these
are the methods of separating hydrogen. The metals of
the alkalies and alkaline earths attack water, forming hydr-
oxides and liberating hydrogen : zNa + 2H9O = 2NaOH +
H2; Ca+ 2H2O = Ca(OH)2 + H9. Magnesium powder,
boiled with water, gives off hydrogen slowly ; but zinc
requires the presence of an acid, and must not be pure,
i.e. there must be a foreign metal present to serve as the
anode. The impurity usually present in commercial zinc
is lead ; the acid, for instance, sulphuric acid, is present in
r + +
dilute solution as ions of HH and SO4 ; the SO4 removes
+ +
the surface layer of the zinc as Zn, while the negative
charge is transferred to the lead, which is in metallic
contact with the zinc. This charge is neutralised by the
» + +
positive charge of the HH, which, on being discharged,
escapes in an non-ionised state. It may then be collected
over water, in which it is very sparingly soluble. Hydro-
gen, while it is on the point of discharging and is still in the
ionised state, may be used to liberate certain elements from
their oxides or chlorides. Zinc and hydrochloric acid,
I- \+~
for instance, in a solution of stannous chloride, SnCl2.Aq,
causes a deposition of tin owing to the exchange of charge ;
the hydrogen retaining its charge instead of parting with it
to the lead or other impurity in the zinc, while the tin is
discharged in its stead. If zinc and hydrochloric acid are
placed in contact with silver chloride, AgCl, which is an
insoluble compound, the hydrogen remains charged, while
the silver parts with the chlorine, the latter remaining in
solution with negative charge. Lastly, if generated in a
solution of ferric chloride, Fe Clg.Aq, the zinc goes into
solution as before ; and the positive electricity is provided
PROPERTIES OF ELEMENTS 25
by the loss of a positive charge provided by the ferric ions
+ + -
changing to the ferrous ions of ferrous chloride, FeCl2. Aq,
+ -
and another molecule of HCl.Aq exists in solution. The
valency of the iron is lowered. Such processes are gene-
rally termed reduction ; the hydrogen is said to be in the
" nascent state/' and is named the "reducing agent."
Metallic iron, manganese, cobalt, and nickel at a red
heat remove oxygen from water with liberation of hydro-
gen: 3Fe + 4H2O = Fe3O4 + 3H2; 2Co + 2H2O = CoO
+ O2. Conversely, a current of hydrogen passed over
these oxides at a red heat will combine with their oxygen,
reducing them to metal. This is an instance of mass-
action. From the equations given above, it is seen that
hydrogen is formed ; it does not remain in the tube to
re-form water ; if it did, there would be a state of balance
or equilibrium, all four substances remaining together in
proportions depending on the temperature and on their
nature ; in the current of steam, however, the hydrogen is
carried on, and is no longer present to act on the oxide of
the metal. And in the converse action the hydrogen
conveys the steam away, so that it can no longer be
deprived of oxygen by the metal.
As already remarked, carbon monoxide has a similar
reducing action on the oxides of the more easily reducible
elements. The product in this case is the dioxide, CO0,
for example, Fe2O3+ 3CO = 2Fe + 3CO2. This action
requires a red heat. Another reducing agent, applied by-
fusing the oxide with it, is potassium cyanide, KCN ;
it is converted into the cyanate, KCNO. The metal
thallium may be prepared by its help, T19O + KCN = 2T1
+ KCNO. As the cyanide is somewhat expensive, it is
used only in special cases.
An instance has already been given of the mutual reduc-
tion of two compounds in the case of nitrogen. Similar
instances are known with lead and with sulphur. The
chief ore of lead is the sulphide, a natural product termed
26 MODERN CHEMISTRY
galena. It is roasted, i.e. heated in contact with air to a
red heat. After a portion has been oxidised to sulphate,
PbS + 2O9 = PbSO4, the temperature is raised, when the
sulphide and the sulphate mutually reduce each other :
PbS + PbSO4 = 2Pb + 2SO2. With sulphur the partial
burning of sulphuretted hydrogen may be explained in a
similar manner; the reaction, 2H2S + O9 = 2H2O + S0,
may be represented as the formation of water and sulphur
dioxide by the complete combustion of one-half of the
hydrogen sulphide, and its reaction with the remaining
sulphide, thus: 2H2S + SO2= 2H2O + 38. And, as a
matter of fact, that reaction does take place on mixing
the two gases in the required proportion of two volumes
of hydrogen sulphide with one of sulphur dioxide.
The Properties of the Elements. — It has been cus-
tomary to divide the elements into two classes, the metals
and the non-metals. As we have seen, this classification
is a completely arbitrary one ; for there are some elements
capable of existing in both states. The name " metal "
was originally given to seven substances, all alike in possess-
ing that bright lustre known as "metallic." These were
gold, silver, mercury, copper, iron, lead, and tin. But in
the Middle Ages bismuth and antimony were isolated in a
fairly pure state, and these, together with zinc, were at first
not received into the class, but were regarded as spurious ;
for they were brittle and easily oxidisable. Although
there is no reason for retaining the division, yet it is often
convenient. Bodies which possess metallic lustre have the
power of conducting electricity better than transparent bodies,
and they are also relatively good conductors of heat.
The elements exist in various physical states. Those
which are gases at the ordinary temperature, however, have
all been condensed to the liquid state by sufficient reduction
of temperature. The lowering of temperature is most easily
produced by means of liquid air, now a cheap commodity.
To liquefy air, it is compressed by a pump to a pressure of
150 atmospheres ; it then traverses a coil of copper pipe,
PROPERTIES OF ELEMENTS 27
and escapes from an orifice at the lower end. Now,
compressed air has some resemblance to a liquid, for
when it expands, as when a liquid changes to gas, heat is
absorbed. The rapidly escaping air becomes cold, and in
passing up over the coil of tube through which it has de-
scended, it cools the pipe, so that the air passing down
becomes colder and colder ; finally, it is so cooled that it
liquefies, and escapes from the orifice in a liquid state. It
may be poured from one vessel to another, with little loss
by evaporation ; and if other gases be allowed to stream
into a tube cooled by its aid, they too are liquefied. The
principle of liquefying hydrogen is the same, for its boiling-
point lies so low that it cannot be liquefied by the aid of
liquid air. That of helium is still lower, but it too has
yielded when compressed into a tube cooled by liquid
hydrogen.
The elements which are gases at the ordinary temperature
are hydrogen, helium, neon, argon, krypton, xenon, nitro-
gen, oxygen and ozone, fluorine, and chlorine. The first
seven are colourless, both in the gaseous and the liquid
state. Oxygen is a colourless gas, but forms a pale blue
liquid ; gaseous ozone has a blue colour ; fluorine is pale
yellow ; and chlorine has a greenish-yellow colour. It
forms a white solid, which, however, melts to a bright
green liquid. Bromine is a dark red liquid at atmospheric
temperature, but above its boiling-point, 59°, it is a deep
red gas. Iodine is a blue-black solid, melting to a black
liquid at 114°, and giving off a violet vapour. Ozone and
the " halogens," as fluorine, chlorine, bromine, and iodine
are called, have all a powerful odour, and act on the skin
in a corrosive manner. Chlorine and bromine are soluble
in water.
Among the other non-metallic elements are boron, a
black, dusty, infusible powder ; carbon, in its ordinary form
an amorphous (i.e. non-crystalline) black substance, of which
the most familiar variety is charcoal ; carbon does not fuse,
but at the enormously high temperature of the electric arc
28 MODERN CHEMISTRY
it volatilises ; silicon, a blackish-brown powder, melting at
bright redness to a lustrous liquid, which solidifies in shining
black lumps ; phosphorus, a waxy, pale yellow solid, melt-
ing at 44.4° ; sulphur and selenium, yellow and brown-red
solids, the former melting at 115° to a brown liquid, and
boiling at 446° ; the latter forming a black liquid at 217°,
and a black vapour at 665°.
The metals of the alkalies, as they are usually called,
lithium, sodium, potassium, rubidium, and cassium, are soft
white metals, at once attacked by water, and oxidised
readily by air, caesium, indeed, taking fire spontaneously.
To protect them from oxidation, they must be kept under
rock-oil or ligroin, a compound which contains no oxygen.
Of these, caesium has the lowest and lithium the highest
melting-point. The metals calcium, strontium, and barium
are sometimes named the " metals of the alkaline earths."
They are hard white bodies, also, like those of the sodium
group, oxidising readily on exposure to air, and at once
attacked by water. Magnesium, zinc, and cadmium are
noteworthy, inasmuch as their temperature of ebullition is
not so high that it cannot be reached in an ordinary furnace;
they can therefore be distilled. Magnesium and zinc are
hard and brittle ; cadmium is softish, like lead, and of a
somewhat greyer tint.
The remaining elements may be classed under the head-
ings, " hard," " soft," " brittle," &c. This implies only
their behaviour at ordinary temperatures ; at higher or
lower temperatures the properties are materially changed.
Mercury, for example, below -40°, is malleable; lead is
brittle.
(a) Malleable metals :—
(1) White^ ductile, moderately hard: — beryllium, alumi-
nium, gallium, indium, tin, silver, nickel. Red, copper.
Tellow, gold.
(2) Grey-white, ductile, and moderately hard: — iron,
manganese, cobalt.
METALS 29
(3) G 'rey- white and soft ; ductile: — thallium, lead ; some-
what harder, and fusible only at a <very high temperature : —
rnodium, ruthenium, palladium, platinum, iridium.
( b ) Liquid metal : — mercury.
(c] Brittle metals : —
(1) White, hard: — antimony, bismuth, tellurium, zirco-
nium, didymium (a mixture), osmium, germanium. Less
hard, arsenic.
(2) Grey, hard : — lanthanum, cerium, yttrium, uranium.
( 3 ) Grey powders, acquiring metallic lustre under the bur-
nisher : — thorium, niobium, tungsten.
(4) Black powders : — tantalum, titanium.
The elements scandium, samarium, and gadolinium have
not been prepared.
Although the external properties of the elements does
not show any obvious relation to their order in the periodic
table (see Part I.), yet it may be generally remarked that
the density increases as each column is descended. Among
the lightest of the elements are lithium, beryllium, magnesium,
and aluminium, at least in the solid state ; whereas osmium,
iridium, platinum, and gold are among the heaviest. But
much more must be ascertained regarding their properties
before a satisfactory comparison can be made.
CHAPTER II
Classification of Compounds — The Hydrides.
Classification of Compounds. — Compounds of the
elements may be divided conveniently into six classes : —
The Hydrides ;
The Halides ;
The Oxides and Sulphides (with Selenides and
Tellurides) ;
The Nitrides and Phosphides (with Arsenides and
Antimonides) ;
The Borides, Carbides, and Silicides ;
The Alloys.
Compounds can be prepared by many methods ; it is not so
easy to classify them as it is to arrange into classes the
methods of preparation of elements. As a rule, the pre-
paration is carried out by one of the following methods : —
(a) The interaction of elements ;
(£) The action of an element on a compound ;
(c) The action of heat on a compound ;
(d) The interaction of compounds ;
(e) The addition of one compound to another.
These methods shall be considered in relation to each of the
groups of compounds named above.
The Hydrides.
(a) The Interaction of Elements. — Lithium, sodium,
and potassium, when heated to 300° in an iron tube in a
INTERACTION OF ELEMENTS 31
current of hydrogen, form white waxy compounds ; that of
lithium has the formula LiH ; as the sodium compound has
the formula Na9H, its existence is difficult to reconcile with
the usual valency of either hydrogen or sodium, for these
elements in all other compounds behave as monads. It would
repay further investigation. It decomposes at 421°.
Iron, nickel, palladium, and platinum, when heated
gently in hydrogen, absorb the gas. Meteoric iron, indeed,
has been known to give off, on heating, 2.85 times its
volume of gas. This natural variety of iron contains about
6 per cent, of nickel. Palladium, gently warmed in an
atmosphere of hydrogen, absorbs over 900 times its volume
of that gas, corresponding to 4.68 per cent, of the weight
of the body produced. It is difficult to determine whether
or not the palladium is in chemical combination with the
hydrogen, or whether the hydrogen is in a state analogous
to solution, for it is known that a solid can exert solvent
power. There is a considerable rise of temperature accom-
panying the absorption ; and if palladium, in a state of
sponge, is placed in contact with a mixture of oxygen and
hydrogen, the mixture may be made to explode. A ther-
mometer-bulb coated with palladium sponge is a good test
for the presence of an explosive mixture of marsh-gas and
air in mines, for the rise of temperature produced is an in-
dication of danger. These metals absorb hydrogen more
readily if they are made the negative electrodes of a
battery with which dilute sulphuric acid is electrolysed.
Iron shows a very curious behaviour under these circum-
stances. If a thin plate of iron is made to close the top of
a barometer-tube full of mercury and a small cell be con-
structed on it, hydrogen will pass through the iron, when
the plate is made the kathode, and will depress the mer-
cury in the tube. No other metal, so far as is known,
shows this peculiarity ; it would appear that the hydrogen
in the ionic state can penetrate the iron.
Carbon, heated to 1200° in an atmosphere of hydrogen,
unites with it to form marsh-gas (methane), CH4. Only
32 MODERN CHEMISTRY
a small percentage of the hydrogen, however, enters into
combination ; a balance soon establishes itself between the
number of molecules of methane being formed and decom-
posed in unit time. At a higher temperature, that of the
electric arc, acetylene, C2H2, is formed, owing to
the decomposition of the methane into that gas and free
hydrogen: — 2CH4 = C9H9 + 3Ht>. Other compounds of
carbon and hydrogen are formed simultaneously, and there
again appears to be a state of equilibrium produced between
the various hydrocarbons formed. With nitrogen, NH,,
it appears to be impossible to induce hydrogen to enter into
direct combination at such temperatures ; but if electric
sparks be passed through a mixture of hydrogen and nitro-
gen, combination to a limited extent ensues. Should the
ammonia, NH3, be removed by having water, or, better,
dilute sulphuric acid, present, the combination proceeds
until all the gases, if they were originally present in the
correct proportion — one volume of nitrogen to two volumes
of hydrogen — have combined. Conversely, if sparks be
passed through ammonia gas, there is nearly, but not quite,
complete decomposition into its constituents. This enables
the volume relations of ammonia to be demonstrated ; for
it is found that two volumes of ammonia gas can be decom-
posed into two volumes of nitrogen and six volumes of
hydrogen. This is symbolised by the equation —
2NH3 = N2 + sH,
Weight 2 ( 14 + 3) 28 3 (2) grams.
Volume 2(22.4) 22.4 3(22.4) litres.
The hydrogen can be nearly completely removed by ab-
sorption with palladium-sponge, and the nitrogen remains.
Water, H20, is more completely formed than any one
of the previously mentioned compounds by the interaction
of its elements. A mixture of oxygen and hydrogen, in
the proportion of one volume of oxygen to two of hydrogen,
is exploded by heat ; this is most easily done by passing an
electric spark through the mixture. While the position of
COMBINATION OF HYDROGEN 33
equilibrium for a mixture of nitrogen, hydrogen, and am-
monia lies at such a point that very little of the compound
is present, but chiefly the uncombined gases, the contrary is
the case with hydrogen and oxygen. Here nearly all the
oxygen and hydrogen combine, and only a trace remains
uncombined. Combination may be made to take place
slowly at much lower temperatures ; even at 300° slow
combination occurs. Colloidal platinum, prepared by mak-
ing an electric arc between poles of platinum under pure
water, which appears to consist of very finely divided
platinum disseminated through the water, has the power of
causing union of oxygen and hydrogen left standing in
contact with it, even at the temperature of the atmosphere.
On the other hand, if water- vapour be raised to a very high
temperature, above 1800°, decomposition into its consti-
tuents takes place with considerable rapidity ; so that it is
possible to obtain a mixture of oxygen and hydrogen by
passing steam through a tube in which a spiral of platinum
wire is kept at a white heat by means of an electric current.
These actions are therefore termed " reversible," and they
are expressed by such equations as —
CH4 ^ C + 2H9 ; 2H9 + O., ^ 2H20 ;
Hydrogen also combines with sulphur when passed
through a flask containing boiling sulphur, and sulphuretted
hydrogen, H2S, decomposes when raised to a low red
heat.
Interesting relations are to be seen with the compounds
of the halogens with hydrogen. In preparing fluorine by
the electrolysis of hydrogen-potassium fluoride, KHF, in
presence of hydrogen fluoride, H2F9, it is possible, by stop-
ping the exit of the hydrogen, to cause a bubble to pass the
bend of the U-tube and to rise into the fluorine ; the instant
the gases unite there is a sharp explosion. This shows
that these gases unite even in the dark to form H2F2.
Chlorine and hydrogen, on the other hand, do not com-
VOL. II. C
34 MODERN CHEMISTRY
bine in the dark, but, when exposed to diffused daylight,
slow but complete combination ensues ; in bright sunlight,
or when illumined by the light from burning magnesium,
the mixture of gases explodes, forming HC1. Bromine and
hydrogen unite to form HBr when a current of hydrogen,
having bubbled through a wash-bottle of bromine, passes
through a red-hot tube ; with excess of hydrogen the union
is practically complete. Iodine and hydrogen, on the
contrary, unite very incompletely to produce HI ; and if
hydrogen iodide be heated, a large proportion of it is
decomposed into hydrogen and iodine. This change has
been investigated much more completely than other changes
of the same character already mentioned ; and as it is
characteristic of all such reversible reactions, we shall con-
sider it in somewhat greater detail.
The rate at which hydrogen iodide is produced from a
mixture of hydrogen and iodine at any constant tempera-
ture is much more rapid than that at which the reverse
change of hydrogen iodide into iodine and hydrogen takes
place. This rate was not difficult to determine. Weighed
quantities of iodine were placed in a tube filled with hydro-
gen, and after heating the sealed tube for a sufficiently long
time for equilibrium to be established, it was opened under
water. The hydrogen iodide formed at once dissolved in
the water, and the residual hydrogen was measured. The
amount of uncombined iodine remaining in the water was
then estimated by known processes. It was thus possible
to find the ratio of the combined to the uncombined hydro-
gen. Now, it was discovered many years ago that the rate
of chemical change depends on the amount of each of the
reacting substances present in unit volume — a condition ex-
pressed by the term "active mass." Thus, if we double
the amount of hydrogen in the mixture of the gases men-
tioned, we double its " active mass/' Let /9 denote the
number of molecules in unit volume of the iodine gas, and
^2 that of the hydrogen, and let ^hi be that of the hydrogen
iodide formed by their interaction. Then, as the rate of
HYDRIDES OF CARBON 35
formation of hydrogen iodide is proportional both to / and to
/>, it will be proportional to their product, h x /. And as
H0 + I9= 2 HI, the rate of change of HI into H2 and I.?
will be 2hi x ihi or 4(/>/)2. If we call the rate of forma-
tion k, and that of decomposition /£', the proportion of
these rates to each other will be kjk' = (h x i)/4(»i)2, if the
gases are present in molecular proportions. At the tem-
perature 440°, and at one atmosphere pressure, it was found
that, taking the total hydrogen as unity, 0.28 was free
and 0.72 combined, after a sufficient time had been al-
lowed for the change to complete itself. Now, the iodine
free must have been equal in number of molecules to the
free hydrogen, i.e. 0.28, and the same number of atoms
of iodine must have existed in combination as of hydro-
gen in combination; hence 0.28x0.28/4(0.72x0.72)
= 0.0375 = ^/y£'. This means that at 440° molecules of
hydrogen iodide decompose into hydrogen and iodine at
a rate only 0.0375 (or one twenty-sixth) of that at which
combination takes place between the two gases.
(b] The action of an element on a compound leads
to the formation of many hydrides. This process has been
pretty fully treated in the description of the methods of
preparation of elements. For example, on passing a current
of hydrogen over hot cupric oxide, water, H.,0, is formed,
while the oxide is reduced to copper, CuO + H9 =
Cu + H2O. The oxides mentioned on p. 16 are thus
reduced. It is not so usual for sulphides to lose sulphur on
heating them in a stream of hydrogen ; indeed, it is only
those sulphides which themselves decompose when heated
that yield to such treatment ; but hydrogen fluoride, chlo-
ride, bromide, and iodide are formed on heating the halides
of many metals in a current of hydrogen. The process,
however^ is not one which is used for the preparation of
these hydrides.
(c) The third method — that of heating a compound —
is also not in use as a means of preparing hydrides^ but it
is often employed in order to produce the compound from
36 MODERN CHEMISTRY
which the hydride is separated. Thus, all compounds
containing water of crystallisation, when heated, lose water
when raised to a high temperature ; and double compounds of
ammonia, too, lose ammonia on rise of temperature. Such
compounds as calcium chloride, CaCl0, crystallise with
water. The formula of the hydrated compound is CaClQ.
6H2O ; a similar compound with ammonia, CaCl0.6NH3,
is also known ; compounds like these lose water or am-
monia when heated. By this plan Faraday succeeded in
liquefying ammonia, which at ordinary temperatures is a
gas. Having sealed up the ammonio-chloride of calcium or
of silver, AgCl.NH3, in an inverted U-tube, one leg was
cooled with a freezing mixture, while the other was heated,
and the gas liquefied under the combined influence of cold
and pressure.
(^/) Most of the hydrides can be prepared by the fourth
method — the interaction of compounds. The decom-
posing agent is either water, an acid, or an alkali.
( i ) Water : — Marsh-gas, CH4, ethylene, C2H4, acety-
lene, C0H0, ammonia, NH3, and phosphoretted hydrogen,
PH3, may be produced by the action of water on some
compounds of carbon, nitrogen, and phosphorus. Alumi-
nium carbide, A14C3, yellow transparent crystals produced
by heating a mixture of carbon and oxide of aluminium to
whiteness in the electric furnace, on treatment with water
yields pure methane, A14C3 + 1 2R,O - 3CH4 + 4A1 ( OH)g.
Manganese carbide, black crystals produced by heating in
the electric furnace a mixture of manganese oxide and
carbon, yields a mixture of equal volumes of hydrogen
and methane, Mn3C + 6H2O = 3Mn(OH)2 + CH4 + H2.
Lithium, calcium, strontium, and barium carbides also
formed in a similar manner in the electric furnace yield
acetylene with water, Li9C.7 + 2H.7O = 2LiOH + C9H9 ;
CaC2 + 2H2O = Ca(OH)2+~C2H2; The carbides of
cerium, CeC2, lanthanum, LaC9, yttrium, YC2, and
thorium, ThC2, yield a mixture of methane, ethylene,
C.7H4, and acetylene, sometimes mixed with hydrogen ;
INTERACTION OF COMPOUNDS 37
and uranium carbide, U2C2, gives methane, ethylene, and
hydrogen, but no acetylene.
Magnesium or calcium nitrides, prepared by heating
metallic magnesium or calcium in a current of nitrogen,
yield ammonia with water: Mg3N2 + 6H2O = 2NH3 -f
3Mg(OH).>, and calcium phosphide, produced by heat-
ing lime with phosphorus, on treatment with water simi-
larly gives off phosphoretted hydrogen : CagP2 + 6H2O =
3Ca(OH)0 + 2PH3. The sulphides of magnesium and
aluminium, MgS and A19S3, are also decomposed by water,
with production of hydrogen sulphide and the hydroxide
of the metal : MgS + 2H.OH = Mg(OH)9 + H2S ; A1.,S3
+ 6H.OH = 2Al(OH)g + 3H2S.
The halides of a certain number of elements are at once
decomposed by water with formation of a hydride of the
halogen and a hydroxide of the element. Boron, silicon,
titanium, phosphorus, sulphur, selenium, and tellurium
chlorides, bromides, and iodides are thus resolved. The
method is practically made use of in preparing hydrogen
bromide, HBr, and iodide, HI, by help of phosphorus.
But the previous preparation of phosphorus bromide or
iodide is unnecessary. It is sufficient to add bromine to
water in contact with red phosphorus, and hydrogen bro-
mide is evolved ; or to warm a mixture of iodine, water,
and red phosphorus. The use of yellow phosphorus is not
advisable, for the action is apt to take place too violently if
it be used. It may be supposed that the phosphorus and
halogen unite to form the pentahalide, which is then imme-
diately decomposed by the water, thus : PBr.(or PI6) +
4H,O = H3PO4+5HBr(or sHI). The gaseous hydride
may be collected over mercury or by downward displace-
ment, or it may be dissolved in water and a solution of
hydrobromic or hydriodic acid prepared.
A commercial method of producing hydrogen chloride,
HC1, depending on the decomposition of magnesium
chloride when heated in a current of steam, has been
patented ; it results in the formation of a compound of
38 MODERN CHEMISTRY
oxide and chloride of magnesium, while the hydrogen of
the water unites with a part of the chlorine ; the resulting
gaseous hydrogen chloride is passed up towers, and comes
into contact with water, thus yielding a solution of hydro-
chloric acid.
(2) In many cases the compound from which the hydride
is formed is not decomposed by water ; an acid, generally
hydrochloric acid, must be present. The reason of this is
+
not easily explained ; it may be that the very few ions of H
and OH present in water are sufficient to effect the decom-
position in some cases and not in others, and that when
an acid is necessary the much larger number of ions of
hydrogen present in its solution is required ; also it is
known that the heat evolved during the decomposition
of those compounds which are altered by water is
greater than that which would be evolved by those which
resist its action were they to be attacked by water.
Many hydrides are prepared by the help of acids. Mag-
nesium boride, Mg3B.,, yields with hydrochloric acid a trace
of BH3 ; but as this compound is a very unstable gas,
almost all of it decomposes into boron and hydrogen. The
similar compound, Mg9Si, produced by heating a mixture
of silica and magnesium powder to redness, when mixed
with hydrochloric acid yields hydride of silicon, SiH4, as
a colourless, spontaneously inflammable gas : — Mg2Si +
4HC1. Aq = 2MgCl2 Aq + SiH4. Arseniuretted hydrogen,
AsH3, and antimoniuretted hydrogen, SbH3, are prepared
from sodium or zinc arsenide or antimonide : Na3 As +
3HCl.Aq = 3NaCl.Aq + AsH3 ; Zn3Sb2 + 6HCl.Aq =
3ZnCl9.Aq + 2SbH3. These gases, however, may be ob-
tained mixed with hydrogen if a solution of oxide of arsenic
or antimony in hydrochloric acid, which yields chloride of
arsenic or antimony, is treated with zinc. The first change
is the replacement of the zinc by the arsenic or antimony,
"
thus : 2 AsCl3. Aq + 3Zn = 3ZnCl2 Aq + 2 As. Electrically
HYDRIDES 39
neutral zinc replaces positively charged arsenic, itself be-
coming positively charged. The arsenic and the unattacked
zinc form a couple, and the hydrochloric acid is electrolysed,
+ - + + ++ - +
2HC1. Aq + Zn = ZnCl2. Aq + 2H ; the hydrogen ion unites
with the arsenic, negatively charged in the electric couple,
forming electrically neutral hydride of arsenic, which escapes
+
as gas, 3 H + As = AsH3. An element in this form, capable
of combination at the moment of liberation, is said to be in the
nascent state, a word derived from " nascere," to be born.
It differs from an ordinary element in being on the point of
losing an electric charge, and it may either be evolved in
+ +
the free state by combining with itself, as H + H = H2, on
giving up its charge, or it may enter into some other form
of combination, as in the case explained. This process of
preparing arsenic or antimony hydride is used as a test for
the elements arsensic or antimony. It was devised by
Marsh, and as the hydrides are very easily decomposed
by a high temperature, the gas, if caused to pass through a
red-hot tube, is decomposed, giving a deposit of arsenic
(grey) or antimony (black). The former is more easily
oxidised than the latter, and dissolves in a solution of
bleaching-powder, in which the latter is insoluble. This
process is particularly applicable where poisoning with
arsenic or antimony is suspected.
H.2S, H2Se, H2Te. — Hydrogen sulphide, selenide, and
telluride are prepared by treating a sulphide, selenide, or
telluride with dilute sulphuric or hydrochloric acid : FeS
+ H9SO4.Aq - FeSO4.Aq + H2S ; Sb0S3 + 6HCl.Aq
= 2SbCl3.Aq + 3H2S. Na2Se.Aq + H2SO4.Aq =
Na2SO4.Aq + H2Se.
Adds. — Hyd'ride of fluorine, chlorine, bromine, and
iodine, when dissolved in water, are termed "acids." As
already mentioned, this name was originally applied to com-
40 MODERN CHEMISTRY
pounds which possess a sharp taste and change the colour
of certain vegetable colouring matters. The word was
later extended to apply to compounds similar in function,
although not acid to taste, which attack the carbonates,
causing them to effervesce, and which yield salts with the
oxides of metals. All acids contain hydrogen, and it is
now possible to define them in a very simple manner. An
acid, in fact, is a compound which yields hydrogen ions
when dissolved in water, or in some other solvent capable
of causing ionisation. This definition applies to the
hydrides of fluorine, chlorine, bromine, and iodine ; and
also to those of sulphur, selenium, and tellurium ; for on
•f- + - +-
solution they ionise thus : HF.Aq ; HCl.Aq ; HBr. Aq ;
HLAq; H.SH.Aq; H.SeH. Aq ; H.TeH.Aq. But
it is not confined to them, for the hydrogen may be united,
not with a simple element, but with a complex group of
+ +
elements, as in H9SO4.Aq or HNO3.Aq. Now, in dilute
solution, a solution of sulphuric acid is less ionised than one
of hydrochloric acid, in about the proportion of 1:2, and
it is therefore a weaker acid ; so that if a hydroxide, such
as sodium hydroxide, be presented to a mixture of equal
numbers of these molecules, in quantity requisite for only
one of them, chloride of sodium will be formed in greater
quantity than sodium sulphate ; yet, on heating a
halide with sulphuric acid, because hydrogen chloride
is a volatile compound, it removes itself from the sphere
of action in a non-ionised state while the sodium
remains as sulphate. Hence these hydrides may be thus
prepared. Hydrogen fluoride, H2F2, is generally prepared by
distilling calcium fluoride, a compound naturally occurring
as " fluor-spar," with sulphuric acid in vessels of lead or
platinum: CaF2 + H2SO4- BaSO4 + H2F2. The use of
lead or platinum is obligatory on account of the action
of hydrogen fluoride on glass or porcelain, the materials
of which flasks and retorts are usually made ; for
HALIDES 41
hydrogen fluoride attacks the silica which they contain,
forming with it silicon fluoride: SiO2 + 2H2F2 = SiF^ +
2H9O. Gold is almost the only other metal which resists
the action of hydrogen fluoride. There is no such diffi-
culty with the other halides. Hydrogen chloride, HC1, is
prepared by distilling from a glass retort a mixture of
common salt and oil of vitriol : NaCl + H.7SO4 = HNaSO4
+ HC1. On a large scale this preparation is carried out
in rotating circular furnaces, the mixture of salt and vitriol
being delivered in through a hopper above, and at the high
temperature the action goes further, and di-sodium sulphate
is produced: 2 NaCl + H2SO4 = NaaSO4+ 2HCL The
gas is passed up towers filled with coke, and exposed to a
descending stream of water, in which it dissolves, forming
a saturated solution of hydrochloric acid, or, as it used to
be called, "muriatic acid" (from "muria," brine).
Hydrogen bromide, HBr, and iodide, HI, may similarly be
produced by distilling together bromide or iodide of sodium or
potassium with exactly the right weight of sulphuric acid for
the equation 2KBr (or zKI) + H2SO4.Aq= K2SO4.Aq +
2HBr (or 2HI). But in these cases, the hydrogen bromide
or iodide is very apt to exert a reducing action on the sul-
phuric acid, depriving it of an atom of oxygen, thus : —
H2SO4 + 2HI - H2SO3 + H2O + I2. Hence it is advis-
abfe to use phosphoric acid, H3PO4, a compound not thus
reduced :— H3PO4 + 2KI = H'K2PO4 + 2 HI.
All these halides come over as gases, and may either be
collected over mercury or by " downward displacement,"
i.e. by delivering them to the bottom of a jar containing
air, which owing to its less density is forced upwards, and
escapes at the mouth of the jar. They cannot be collected
over water, for they are readily soluble in it.
The compound HN3, termed hydrazoic acid (from the
French term for nitrogen, "azote"), is also liberated in
the gaseous form by warming its sodium salt with sulphuric
acid. It, too, is readily soluble in water.
(3) Certain hydrides are set free by the action of an
42 MODERN CHEMISTRY
alkali, i.e. the hydroxide of one of the metals of the sodium
or the calcium group. It is true that the change may be
produced by other hydroxides, but they are not so efficient,
and not so generally employed. Among these are ammonia,
NHg, and hydrazine, NJH4. These bodies unite with
acids ; for example, ammonia and hydrogen chloride form
ammonium chloride, NH4C1, when mixed: — NH3 + HC1
= NH4C1. This compound is produced by a change in
valency of the nitrogen atom ; in ammonia it is a triad, N'",
but on union with hydrogen chloride the valency of the
nitrogen becomes five, Nv. On distillation of a mixture of
ammonium chloride with caustic soda or with slaked lime,
either in presence or absence of water, the following change
occurs :— NH4C1 + Na O H = NaCl + NH3 + H2O ;
2NH4Cl + Ca(OH)2 = CaCl2 + 2NH3+2H2O. The
initial change is the formation of ammonium hydroxide,
NH4OH ; this substance, being unstable when heated,
decomposes into ammonia and water. Hydrazine, a com-
pound of the formula N0H4, is similarly liberated from its
chloride.
The usual source of commercial ammonia is coal-gas.
On distillation of coal, all varieties of which contain nitrogen,
it may be imagined that when methane, the principal consti-
tuent of coal-gas, is strongly heated it splits into carbon and
hydrogen. This hydrogen, at the moment of its formation,
is in the nascent state, and it unites with the nitrogen, which
is also in the nascent condition. As ammonia is very easily
soluble in water, while the other constituents of coal-gas are
sparingly soluble, the gas is deprived of ammonia by passing
it through " scrubbers," pipes containing broken bricks kept
moist with water. The ammonia dissolves, while the coal-
gas passes on. The solution is next mixed with hydro-
chloric acid and evaporated to dryness. The residue of
ammonium chloride is then distilled with lime, as previously
described. The ammonia is received in water, and brought
into the market in the form of a concentrated solution, to
which the name " liquor ammoniac " is given.
PROPERTIES OF HYDRIDES 43
(b) Certain double hydrides are formed by the addition
of one hydride to another. Ammonia and hydrazine unite
with hydrides of the halogens to form salts, such as ammo-
nium chloride, NH4C1 ; but as these bodies show analogy
with salts of the metals, they will be reserved until the latter
are considered.
General Nature of the Hydrides. — Hydrides of
lithium, sodium, potassium, iron, nickel, palladium,
and platinum differ from the others in character ; they are
solid bodies, decomposed by heat. Graham, indeed, who
investigated that of palladium, was struck with the metallic
nature of the substance, and was inclined to believe that it
might be regarded as an alloy of a metallic form of hydrogen,
to which he gave the name " hydrogenium ; " and it was for
long believed that liquid hydrogen would show the character-
istic property of metals, metallic lustre. But this anticipation
has not been fulfilled. Liquid hydrogen is a colourless body ;
and solid hydrogen is described as having a white crystalline
appearance, like ice froth. But it must be confessed that
hydrogen shows a marked similarity to metals in many of its
compounds, as will be frequently seen in the sequel.
The remaining hydrides may be divided into three
classes : — Those which react with neither acid nor bases,
and which may therefore be described as neutral. To this
class belong the hydrides of boron, carbon, silicon,
arsenic, and antimony. That of phosphorus nearly falls
into the same category, for its compounds with acids are
very unstable. The next class — those which react with
bases — comprises water and the hydrides of sulphur,
selenium, and tellurium. The compounds are termed
hydroxides, or, in the case of sulphur, hydrosulphides.
These will be considered later, but an instance may be given
here : — When lime is moistened with water it is slaked,
with formation of calcium hydroxide, thus : CaO + H2O =
Ca ( OH ) 2. The hydrides of fluorine, chlorine, bromine,
and iodine also belong to this class ; but in their case an
exchange takes place, thus : CuO + 2HCl.Aq = CuCl2.Aq
44 MODERN CHEMISTRY
= H9O. Hydrazoic acid is capable of similar reactions.
Such hydrides, with the exception of water, are generally
termed acids. The last group of hydrides, ammonia and
hydrazine, and, in one or two isolated cases, hydrogen phos-
phide, unite with acids, forming salts, thus : NH3 + HC1 =
NH4C1; PH3 + HI = PH4I. It appears that the pre-
sence of water is necessary for at least the first of these
combinations ; for if perfectly dry hydrogen chloride is
mixed with perfectly dry ammonia, no combination results.
It is perhaps allowable to suppose that the presence of
moisture leads to ionisation of the hydrogen chloride, and
that the ionised molecule is capable of entering into com-
bination, while the non-ionised molecule is without action
on the ammonia. These compounds will be treated of
under the heading of " salts."
The hydrides of boron, carbon, silicon, phosphorus,
arsenic, and antimony are insoluble in water ; those of
nitrogen, sulphur, selenium, tellurium, and the halo-
gens are soluble. With the exception of certain hydrides
of carbon, to be afterwards described, and water, all the
rest are gases at atmospheric temperature. The fact that
water is a liquid, and not, as might be expected, a gas, re-
quires comment. It is noteworthy that water-gas possesses
the density 9, corresponding to the molecular weight 1 8 ;
hence there can be no doubt that in the gaseous state water
has the formula H9O. But it is known that compounds of
sulphur, which are in formulae, and in many properties
analogous to compounds of oxygen, possess higher boiling-
points than the corresponding oxygen compounds. For
instance, bisulphide of carbon, CS2, boils at 44°, whereas
carbon dioxide boils at about —80°. But water boils at
100°, and, contrary to expectation, its analogue, sulphuretted
hydrogen, condenses to a liquid at a temperature much
below o°. Now, it has been found by a method depend-
ing on the rise of liquids in capillary tubes, that while the
molecular weight of most substances in the state of liquid is
identical with those which they possess in the gaseous state,
HYDROCARBONS 45
the molecular weight of water is considerably too great.
The conclusion follows, therefore, that the molecular
weight of water should be expressed by a more complex
formula than H2O ; possibly by H4O2, or by one even
more complex. Gaseous hydrogen fluoride, unlike its
congeners, has a higher molecular weight than that ex-
pressed by the formula HF ; determination of its density
leads to the formula H2F2. These facts are probably to be
explained by the view that oxygen may possess a higher
valency than 2, and fluorine than I, at relatively low tem-
peratures. It is not unlikely that the structural formula of
H\ /H
liquid water is >O=O< , and that of hydrogen
H/ \H
fluoride HF=FH, where oxygen acts as a tetrad and
fluorine as a triad.
Hydrocarbons. — The hydrides of carbon, or
" hydrocarbons," are very numerous, and form an im-
portant group of substances. In many respects they are
analogous to the metals, and they yield derivatives com-
parable with those of the metals. The preparation of some
of them has already been described ; but in order to give a
more complete idea of their structure and functions, a short
description of other methods of forming them is annexed.
Methane or marsh-gas, if mixed with its own volume of
chlorine, and exposed to daylight — not sunlight, else the
mixture would explode — undergoes the reaction CH4 +
C12 = CH3C1 + HC1. The resulting gas, termed chloro-
methane, is soluble in ether, a volatile liquid compound
of carbon, hydrogen, and oxygen. If pieces of metallic
sodium are added to the solution, the sodium withdraws
chlorine from the chloromethane and a gas is evolved.
On analysis, it gives numbers answering to the formula
CH3. But if that were its formula, its molecular weight
in grammes would occupy 22.4 litres; but 15 grammes
occupy only 11.2 litres; hence its molecular weight must
be 30, and not 15, and its formula cannot-be CH3, but must
46 MODERN CHEMISTRY
be C2H6. It is reasonable to suppose that the mechanism of
H\ : ; /H
the reaction is this : H— ,C— :C1 + Na Na + Cl— C— H ;
__ H/ \H
and that the two CH3 groups on liberation join together,
H\ /H
forming the complex group, H— ~C — C — H. Similarly,
H/ \H
mixing C9H6, which is named ethane, with its own
volume of chlorine, a reaction takes place like that with
methane, and chlorethane is formed, thus : C2H6 + C12 =
C9H5C1 + HC1. Chlorethane dissolved in ether and treated
with sodium yields not C2H5 but C4H10, and it may be
supposed that the constitution of the new hydrocarbon,
H H H H
butane, is HC — C — C — CH. A mixture of chloro-
H H H H
methane and chlorethane gives with sodium an intermediate
H H H
hydrocarbon, CJHL, propane, HC — C — CH. When
H H H
chlorine and propane are mixed in equal volumes, two
chloropropanes result; they have identical formulas and
molecular weights, and it is believed that the difference
between them consists in the position of the entering atom
of chlorine. In one case the chlorine replaces hydrogen
attached to one of the terminal atoms of carbon, thus :
H H H
Cl C — C — CH, while in the other the medial hydrogen is
H H H
H Cl H
replaced : HC — C — CH. These two chloropropanes
H H H
yield in their turn two methylpropanes or butanes.
Two such substances are said to be isomeric, or to exhibit
isomerism with each other. The following list gives the
names and formulae of some of this series of hydrocarbons ;
HYDROCARBONS
47
where the difference between their formulae is CH2, they
are said to form a " homologous series."
H
HCH
H
Methane.
H H H
HC— C— CH
H H H
Propane.
H H H
HC— C— CH
H | H
HCH
H
Isobutane.
H
HCH
| H H
HC— C— CH
| H H
HCH
H
Isopentane.
H H
HC— CH
H H
Ethane.
H H H H
HC— C— C— CH
H H H H
Butane.
H H H H H
HC— C— C— C— CH
H H H H H
Pentane.
H
HCH
H | H
HC— C— CH
H | H
HCH
H
Tetramethyl-methane.
Chloromethane, if mixed with its own volume of chlorine
and exposed to light, yields a dichloromethane, thus :
CH3C1 + C12 = CH?C12 + HC1. This compound, which,
like chloromethane, is also a gas soluble in ether, on treating
its solution with sodium, loses chlorine and is converted
into ethylene, thus: CH9C19 + 4Na + C19CH9 = 4NaCl
H H
+ C=C . The carbon atom, it will be observed, is still
H H
a tetrad, but the two atoms are connected by a " double
48 MODERN CHEMISTRY
bond." Homologues of ethylene are known, of which the
following are a few : —
H H H H H
C=C HC— C=C
H H H H
Ethylene. Propylene.
HHHH HHHH H H
HC— C— C=C HC— C=C— CH HC— C— CH
HHHH H H || H
HCH
Butylenes.
These hydrocarbons are characterised by the facility with
which they combine with the halogens, forming oils ; they
have, therefore, been termed "defines," or " oil-makers.'*
They also unite with nascent hydrogen, and are converted
into paraffins, as the members of the former group are
termed. The equations which follow illustrate this : —
H2C Cl H9CC1 CH., CH3
1+1=1 || " + 2tt= |
H2C Cl H,CC1 CH2 CH3
By the further action of chlorine on dichloromethane,
trichloromethane, or chloroform, CHC13, is produced.
Chlorine can also be withdrawn from chloroform by sodium,
and acetylene, C2H2, is formed: HCCl3 + 6Na + Cl3CH
= 6NaCl + HC=CH. Here the two carbon atoms are
represented as united by a treble bond, and each carbon
atom is still believed to remain tetrad. Acetylene is
also characterised by the ease with which it unites with
chlorine, forming a tetrachlorethane : HC^CH + 2C12 =
C19HC— CHC13. Here, also, other members of the series
are known.
The passage of acetylene through ^ red-hot tube is
attended by " polymerisation ; " that is, two or more mole-
cules unite to form a more complex one. In this case, three
HYDROCARBONS 49
molecules of acetylene combine to form a molecule of the
formula C6H6, a compound to which the name benzene is
applied. It is produced in large quantity by the distillation
of coal, and is separated from coal-tar oil by distillation.
Its carbon atoms are imagined to form a ring, because,
among other reasons, it yields only one mono-chloro-sub-
H H H
C— C=C
stitution product : || gives on treatment with
C— C=C
H H H
H H Cl
C— C=C
chlorine, C19, || | ; and as all the hydrogen atoms
^ c_ c=c
H H H
in the molecule are symmetrically arranged with respect to
the carbon atoms, this condition is fulfilled.
The four first members of the methane series are gases ;
those containing a greater number of atoms of carbon up
to eleven are liquids, and the higher members are solids.
The paraffin oil which is burned in lamps consists of a
mixture of the liquid members, and paraffin candles largely
consist of the solid members. They are all practically
insoluble in water. The olefines have similar physical pro-
perties, and benzene is a volatile liquid. Iodine, sulphur,
and phosphorus dissolve in the liquid hydrocarbons.
These and other hydrocarbons may be considered as
somewhat analogous to the metals ; the analogy appears in
the methods of formation and formulas of their derivatives.
CHAPTER III
The Halides of the Elements— Double Halides
— Endothermic Combinations — Hydrolysis
— Oxidation and Reduction — Mass=Action.
The Halides. — Compounds of fluorine, chlorine, bro-
mine, and iodine are thus named. They fall into classes
when the elements are arranged according to the periodic
system. Taking the chlorides as typical of the halides,
we have the following table : —
LiCl
NaCl
BeCl2
MgCl2
BC13
A1C13
CC14
SiCl4
NC13
PC15 PC13 SF6
sci4'
OC12
SC12
HC1
... FC1?
... C1C1
KC1
RbCl
CsCl
CaCl2
SrCl2
BaCl2
ScCl3
YC13
LaCl3
TiCl4
ZrCl4
CeCl4
ThCl4
... AsCl3 ...
SbCl5 SbCl3
ErCl3 ...
BiCl3
SeCl4
TeCl4
TeCl2
IC13 IC1
CuCl
AgCl
ZnCl,
CdCl2
GdCl,
HgCf2
GaCl3
InCl3
Tici3
GeCl4
SnCl4
TbCl4
PbCl4
VC15 VC1,
NbCl5NbCl3 ...
... PrCU ...
TaCl3 ... WC16
MoCl4
WC14
CrCl2
MoCl2
NdCl2
WC12
MnCl3 ...
FeCl3 Fed.,
RuCl3 RuCl2
CoCls CoCl2
... RuCl3 ...
PdCl4
NiCl2
PdCU
OsCl4 OsCl3 OsCl.2 IrCl4 IrCl3 ... PtCl4 ... PtCl2
50
THE HALIDES 51
Besides these compounds, which present considerable
regularity, others exist which have less claim to order.
Thus, KI3 is also known ; it is unstable, but CsI3 is re-
latively stable. Again, CuCl2 and AuClg exist, also
HgCJ. In the next group, GaCl9, InCl, and InCl2 are also
known, as well as T1C1. The following group contains
SnCl., and PbCl2 ; PbCl4 is very unstable. Besides
VCl/and VC13, VC14 and VC12 are also known ; and in
the next group, CrCl3, MoCl3, and MoCl5, also WC15,
UC13, and UC15. These compounds are difficult to
classify.
The bromides and iodides, as well as the fluorides, corre-
sponding to many of these chlorides in formula, are also
known. Where they are of special interest, they will be
alluded to in the sequel.
The characteristic of the halides of the elements of
the lithium group is that they are all soluble white salts,
crystallising in cubes. In dilute solution they are all ion-
ised, and even in strong solution a large percentage of ions
are present. Hence they all react as metal ions and as halo-
gen ions. Thus, for instance, with silver nitrate, which is
the usual test for ionic chlorine, the following reaction takes
place : — NaCl. Aq + AgNO3. Aq = NaNOg. Aq + AgCL
Practically insoluble, and therefore practically non-ionised,
silver chloride is precipitated, and free ions of sodium and
the nitrate group remain in solution. If concentrated
solutions are mixed, that portion which is ionised reacts ;
and as it is removed from solution, the originally non-
ionised molecules of sodium chloride are ionised, because
the solution becomes more dilute as regards sodium chloride,
and they, too, enter into reaction. In a similar way, the
alkali metal ions react in presence of a suitable reagent.
Another point to be noticed is that these salts are not
hydrolysed, that is, do not react with water to give hydroxide
and acid to any appreciable extent, and the usual method
of preparing them depends on these facts. They may
52 MODERN CHEMISTRY
all be obtained by the addition of halogen acids to the
hydroxides or carbonates of the metals dissolved in water,
thus: KOH.Aq + HBr.Aq-KBr.Aq + H2O. It will
be noticed that the water is not ionised, nor does it hydro-
lyse the potassium bromide ; hence, on evaporation, as
concentration increases, the number of ions of potassium
and bromine becomes fewer and fewer, and after the
water has been removed the pure dry salt is left. With
a' carbonate the action is similar. The equation is :
U2CO3.Aq + 2HI.Aq = 2LiL Aq + H2O + CO2. In
dilute solution the acid H2CO3 would be liberated ; it is a
very weak acid, /'.<?. it is comparatively very slightly ionised into
+
2H.Aq and CO3.Aq ; and, moreover, it readily decom-
poses into H2O and CO2 ; hence it is removed from the
sphere of action as it is formed, and on evaporation the
salt is left behind, as in the previous example.
Sodium and potassium chlorides occur in nature ; the
former in the sea, which contains from 3.8 to 3.9 per cent.
Deposits, which have undoubtedly been formed by the drying
up of inland seas, are found in many places. At Stassfurth in
S. Germany there are large deposits of all the salts present in
sea- water, including common salt, chlorides and sulphates of
magnesium, potassium, and sodium, and calcium sulphate ;
these have been deposited in layers in the order of their
solubilities, the less soluble salts being deposited first.
Bromides and iodides are also present in minute quantity in
the residues from the evaporation of sea- water.
Solutions of the halides of the beryllium group of elements
can also be made by acting on the hydroxides or carbonates
of the metals with the halogen acid. To take barium chlo-
+ +- + - ++-
ride as an example, BaCO3.Aq+ 2HCl.Aq = BaCl2.Aq
+ H2O + CO2. Now barium carbonate is nearly insol-
uble in water, but the portion which dissolves is ionised ;
and, as explained above, when the portion which is ionised has
THE HALIDES 53
reacted, its place is taken by more of the carbonate entering
into solution ; so that finally all is changed into chloride.
With the hydroxides, the same kind of reaction takes place :
Ca(OH)9.Aq + zHBr.Aq == CaBr^Aq + 2H,O.
These salts are also white and soluble in water. There is,
however, one exception, namely, calcium fluoride, CaF9,
which occurs native as fluor- or Derbyshire spar. It forms
colourless cubical crystals, and is the chief compound of
+ + -
fluorine. It is produced by precipitation: CaCl9.Aq +
q = CaF2H-2KCLAq. The calcium fluoride is
non-ionised, and comes down in an insoluble form.
Water of Crystallisation. — The other halides of
this group crystallise with water of crystallisation ; its
amount varies from 7 molecules, as in BaI9.yH9O, to I as in
ZnCl9.H9O. The retention of this so-called " water of
crystallisation " has not yet been satisfactorily explained.
It was for long believed that such compounds were " mole-
cular/' as opposed to atomic ; that is, that the water
molecules combined as wholes with the salt, and not by
virtue of their atoms ; but it is more probably to be explained
by the tetravalency of oxygen, although even with this
assumption it is not easy to ascribe satisfactory constitutional
formulas in all cases. It must at the same time be assumed
that the halogen atoms are of a higher valency than unity ;
possibly triad, or even pentad.
These salts are hydrolysed in solution to a small extent ;
thus a solution of magnesium chloride, besides containing a
large number of ions, has also reacted with the water to form
hydroxide and hydrogen chloride: MgCl9 + 2H(OH)
= Mg(OH)2 + 2HC1. As the solution becomes con-
centrated on evaporation, the hydrogen chloride volati-
lises with a part of the water ; and a mixture, or rather a
compound, of the oxide and chloride remains. Hence these
chlorides cannot be obtained in a pure state by evaporating
their solutions. They exhibit another property, however,
54 MODERN CHEMISTRY
which makes it possible to obtain them in a pure state,
namely, the power of forming " double halides." This pro-
perty is not well marked with the halides of calcium, barium,
and strontium, but the halides of beryllium, magnesium,
zinc, and cadmium are notable in this respect. We have,
for example, MgCl2.KC1.6H2O, ZnCl2.NH4Cl, and many
similar bodies. In solution, such compounds are mainly
ionised into their simple ions, but on evaporation the non-
ionised salt separates in crystals, and is not subject to
hydrolysis. Hence such salts can be dried without decom-
position. The ammonium salts, when sufficiently heated,
lose ammonia and hydrogen chloride by volatilisation, and
the anhydrous halide is left]: MgCl2.NH4Cl = MgCl2 +
NH3 -f HC1. The mode of combination of these double
salts is possibly owing to the fact that the halogens are
capable of acting as triads ; thus Zn/ may be
taken as the constitutional formula of that particular salt.
The mono-halides of copper, silver, and gold may be
attached to the first group ; and if that is done, the mono-
halides of mercury must also be included. These com-
pounds are all insoluble in water, and are consequently
obtained by precipitation or by heating the higher halides,
where these exist. Thus CuCl and AuCl are obtained
by cautiously heating CuCl2 and AuCl3 ; they are white
insoluble powders. Cuprous chloride is more easily
obtained by removing half the chlorine from cupric chloride
dissolved in concentrated hydrochloric acid, by digesting
it with metallic copper: CuCl2.2HCl.Aq + zHCl.Aq
+ Cu = Cu2Cl2.4HCl.Aq, a brown compound, which is
decomposed by water into Cu9Cl9 and 4HCl.Aq ; the
cuprous chloride is thrown down as a snow-white powder.
With silver and mercury, the chlorides AgCl and HgCl are
formed by precipitation from the respective nitrates, AgNO3
and HgNO3, on addition of soluble chlorides. The bromides
and iodides are similarly formed, and are also insoluble.
THE HALIDES 55
There are several interesting points connected with these
halides. First, as regards their colour; the chlorides are
white; cuprous bromide is greenish brown, while the brom-
ides of silver, gold, and mercury are yellow ; and cuprous
iodide is brownish, and the iodides of the other metals darker
yellow than the bromides. It appears as if the colour was
influenced both by the- metal and by the halogen. Next,
the chlorides of copper and mercury give evidence of
possessing the double formulae Cu2Cl2 and Hg2Cl2, which
would imply that the metals were only pseudo-monads,
and that the structural formulas should be Cl— Cu— Cu— Cl
and Cl— Hg— Hg— Cl ; and this would correspond with
the fact that the chlorides CuCl2 and HgCl2 are also
known ; but, on the other hand, as AgCl in the state of gas
has the simple formula given to it, it may be that it is the
halogen which forms the bond of union between the two
half- molecules, thus : CuCl=ClCu. Silver forms no higher
halides.
The fluorides of these elements differ from the others in
being soluble in water ; they are prepared from the oxides
with hydrofluoric acid. They are very difficult to dry,
for they undergo the reverse reaction, and are hydrolysed
into oxide and hydrogen fluoride on evaporation.
Copper and mercury also function as dyads ; that is,
their ions are capable of carrying a double electric charge
under certain circumstances. What the mechanism of this
change is, we do not know ; but the change in valency can
be induced by presenting to the element a larger amount of
halogen, if it is desired to increase the valency, or by remov-
ing halogen if the opposite change is required. The addition
of halogen to the mono-halide is in each case an exothermic
change, and its converse is an endothermic one. Cuprous
or mercurous chloride, heated in a current of chlorine
changes to cupric or mercuric chloride, and the converse
change can be brought about by heating the higher halide
in a current of hydrogen, or by exposing the lower halide
to the action of nascent hydrogen ; but it is difficult to
56 MODERN CHEMISTRY
prevent the action in the latter case from going too far and
yielding the metal. A solution of cupric chloride saturated
with sulphurous acid in presence of hydrochloric acid, and
then diluted with water, gives a precipitate of cuprous
chloride : 2CuCl2. Aq + H2SO3. Aq + 2HC1. Aq + H2O =
Cu2Cl2.4HCl.Aq + H9SO4.Aq. The sulphurous acid re-
moves oxygen from water, liberating hydrogen in presence
of the cupric chloride, and the latter is deprived of half
its chlorine and reduced to cuprous chloride. Similarly,
stannous chloride forms a reducing agent for mercuric
chloride : 2HgCl2. Aq + SnCl2. Aq = Hg2Cl2 + SnCl4. Aq.
The converse change can be produced by exposing the lower
halide in presence of halogen acid to the action of nascent
oxygen : Cu2Cl2 + zHCl.Aq + O - iCuCL,. Aq + H2O.
This oxygen in the case of copper may be molecular, O9,
but for the formation of the higher halide of mercury, it
must be derived from some substance capable of parting
readily with oxygen, such as nitric acid.
Cupric iodide is very unstable, and readily yields up
iodine, forming cuprous iodide. On mixing cupric chloride
with potassium iodide, the cuprous iodide is precipitated :
2CuCl2. Aq + 4-KI. Aq - Cu2I2 + 4KCl.Aq + I2. It is to
be noticed that the dyad cupric ions have lost two charges,
ancl that these have neutralised the two negative charges of
the iodine ions, causing them to be precipitated. (Inasmuch
as the cuprous iodide is insoluble, it should not have had the
ionic signs attached ; but they have been kept in order to
show the changed valency. ) Mercuric iodide is an insoluble
scarlet precipitate, and is therefore best produced by pre-
cipitation. It dissolves, however, in a solution of potassium
iodide, forming a double salt, of which more shortly.
Auric chloride contains triad gold, and thus has the
formula AuCl3. It is not produced by the direct action
of chlorine on gold, because the temperature of attack is
above the temperature at which the compound is decomposed.
But it is possible to volatilise gold in a current of chlorine,
THE HALIDES 57
because a few molecules escape decomposition and are
volatilised along the tube through which the chlorine is
passed, and on cooling the gold is deposited, owing to the
decomposition of the chloride at a lower temperature. It
may appear paradoxical that the chloride is stable at a
higher temperature than that at which it decomposes ; but
it is to be presumed that the difference of temperature
between one favourable to an exothermic and to an endo-
thermic action is very small ; and as endothermic substances
increase in stability on rise of temperature, the chloride is
capable of volatilisation ; on cooling it becomes unstable and
undergoes decomposition with deposition of gold. The
usual method of preparing this salt is to dissolve gold in
a mixture of nitric and hydrochloric acids. This mixture
yields ionic chlorine, the negative charge of which neutralises
the positive charges of the gold ; but there are corresponding
negative charges set free, which are transferred to the ion
NO3 of the nitric acid, converting it into 2O, with its four
negative charges. The latter combines with the hydrogen,
+ - + -
forming electrically neutral water: 3HC1 + HNO3.Aq +
Au = 2H2O + Au C13 . Aq + NO.
Auric chloride forms dark red crystals ; it is soluble in
water, and when mixed with chlorides of the alkali metals
forms a set of salts termed aurichlorides. The potassium
salt, for example, has the formula K AuCl4 ; it is soluble in
water, but, unlike the "double salts/' such as MgCl2,2KCJ,
already alluded to, it is ionised by water, not into simple
+
ions like these, but into the ions K and the complex group
AuCl4. At the same time there exists in the solution a small
number of simple ions, so that on electrolysis gold is deposited
at the kathode, but the primary effect of the current is to
send the aurichloric ions to the anode. The solution of mer-
curic iodide in potassium iodide, of which mention was
madebefore,is a half-way example of the same kind. Its solu-
58 MODERN CHEMISTRY
+
tibn contains ions of K and HgI3, but these are mixed with
+
a much larger proportion of the simple ions, K and I and
i- +
Hg and I2. Ail grades of such salts are known ; indeed it
is probable that the double salts, such as magnesium-potassium
chloride, contain a small number of complex ions of MgClg.
These halides have been considered at length because
they form types of the others. Use will be made of the
examples given in treating of the remaining halides.
We have seen that the halides may undergo either ionisa-
tion or hydrolysis, or both at once. The ionisation may be
more or less complete, and the hydrolysis is promoted by
dilution and by a high temperature. The remaining halides
display both these kinds of behaviour, and according as one
or the other prevails, the methods of preparing them are
affected. In certain cases, moreover, the halides form
compounds with other halides, usually those of the alkali
metals or hydrogen, which are less apt to be hydrolysed,
and yield different complex ions. The halides of carbon and
nitrogen belong to neither of these classes, for they are
insoluble in and unacted on by water. As neither carbon
nor nitrogen is acted on by the halogens (excepting that
carbon burns in fluorine), they must be prepared indirectly
by acting on one of their compounds with the halogen.
Methane or carbon disulphide is chosen for the former, and
ammonia in preparing the latter. By passing a current of
chlorine saturated with the vapour of carbon disulphide
through a red-hot tube, the chlorides of both carbon and
sulphur are formed: CS2 + 3C12 = CC14 + S2C12. On
treatment with water the sulphur chloride is decomposed,
while the chloride of carbon may be distilled off ; it forms
a colourless liquid boiling at 76.7°. Its smell resembles
that of the closely allied chloroform, CHCJ3, and it is also
possessed of anaesthetic properties. For the preparation of
nitrogen chloride a jar of chlorine is inverted over a
THE HALIDES 59
saturated solution of ammonia in water ; oily drops are
formed which settle to the bottom of the vessel : NH3. Aq +
3C12 = NC13 + 3HCl.Aq ; the HC1 unites with ammonia,
forming ammonium chloride.
Endothermic Combination. — This body is fearfully
explosive, for its formation is attended by great absorption
of heat ; but during its formation the reagents do not grow
cold ; for the formation of ammonium chloride is a highly
exothermic reaction, and the amount of heat evolved by
its formation is greater than that of the equivalent amount
of chloride of nitrogen ; hence the change as a whole is
accompanied by evolution of heat. It is thus that endo-
thermic compounds are usually formed : by virtue of a
simultaneous action in which heat is evolved. The slightest
shock causes the decomposition of such endothermic bodies ;
if one single molecule is decomposed, it evolves heat and
brings about the decomposition of its neighbours ; and as all
the molecules are in close proximity to each other, and as
the products, nitrogen and chlorine, are both gases, and are,
moreover, much raised in temperature by being set free, the
decomposition is accompanied by sudden and enormous ex-
pansion. Nitrogen iodide, prepared by adding a solution
of iodine to aqueous ammonia, is a black solid of the
formula NI3.NH3 ; it is also explosive.
The fluorides of boron and silicon are both produced
by the action of a strong solution of hydrofluoric acid on the
oxides ; but it is necessary to have some agent present
to withdraw water, such as concentrated sulphuric acid.
These compounds are both gaseous. Their formation is
shown by the equations: B2O3 + 6HF = 2BF3 + 3H2O ;
SiO2 + 4HF = SiF4+ 2H3O. If the water is not with-
drawn, combination ensues between the fluoride and hydrogen
fluoride, with formation of HBF4 or H9SiF6, named re-
spectively hydroborofluoric, and hydrosilicifluoric acids
thus:
2H2SiF6 + H2SiO3. These compounds ionise into H-
60 MODERN CHEMISTRY
ions, and the complex ions BF4 and SiFfi ; and many salts
are known in which metals replace the hydrogen. They
are similar in kind to potassium aurichloride.
The other halides of boron and silicon, and also of
phosphorus, sulphur, selenium, tellurium, and iodine,
react at once with water, forming hydrogen halide and an
acid. The equations are as follows : —
BCl3+3H9O.Aq = B(OH)3.Aq+3HCl.Aq;
SiCl4+3H;O.Aq - O=Si(OH)9 + 4HCJ.Aq;
PCJ3+3H9O.Aq = P(OH)3.Aq+3HCl.Aq;
4H;O.Aq = 0=P(OH)3.Aq+5HCl.Aq;
2 + 2H0O.Aq = O=S(OH)2.Aq
"
9O.Aq:= O=Te(OH)2.Aq + 4HCl.Aq
+ Te;
+ 3H9O.Aq = O=S(OH)2.Aq + 4HCl.Aq;
H2O.Aq = 3HIO3.Aq+i5HCl.Aq + I2.
It is to be noticed that where a hydroxy-compound
corresponding to the halide is capable of existence, it is
formed ; if not, excess of the element is set free. Hence
none of these halides can be prepared by acting on the
hydroxide with a halogen acid ; they are all made either
by the direct action of the halogen on the element, or by
what comes to the same thing, the action of the halogen on
a strongly heated mixture of the oxide of the element with
carbon. Boron, silicon, and phosphorous chlorides are vola-
tile liquids ; they fume in the air owing to their action on
the water-vapour. S9C12 is a yellow liquid ; when saturated
with chlorine at a low temperature, SC19 and SC14 are
successively formed ; but on rise of temperature they dis-
sociate into the lower chloride. IC1 is a black solid,
converted by excess of chlorine at a low temperature
into IC13, a yellow solid, which easily dissociates into
IC1 and C19 ; and PC15 is a pale yellow solid, volatile at
a high temperature in a perfectly dry atmosphere without
THE HALIDES 61
dissociation, but resolved by the least trace of moisture into
PC13 and C12.
Valency of Elements. — We may remark here the
gradual increase of valency as we pass from left to right in
the periodic table. Lithium is a monad, with its congeners ;
the elements of the beryllium group are dyads ; boron a
triad ; carbon a tetrad ; phosphorus acts as pentad as well
as triad ; sulphur, as a pseudo-monad, a dyad, and a tetrad ;
and Moissan has lately shown that sulphur burns in fluorine,
forming a very stable hexafluoride, SFC ; while iodine forms
a monochloride and a trichloride, and probably also a
pentafluoride and a heptafluoride.
Passing back to the boron group, if it is desired to form
anhydrous chloride, it is necessary either to heat the element,
or its oxide mixed with charcoal, in a current of chlorine,
or, except in the case of boron, to prepare a double salt of
the chloride with ammonium chloride, and to volatilise the
latter after driving off the water ; the aqueous chlorides
are formed by dissolving the oxides or hydroxides in
hydrochloric acid. Thallium forms monohalides, sparingly
soluble in cold water, and thereby attaches itself to the
copper group.
Almost the same remarks apply to the elements of the
carbon group ; solutions of the chlorides, with exception
of those of carbon and silicon, are obtained from the
element and hydrochloric acid or from the hydroxide, and
they cannot be dried without reacting wholly or partially
with water. For instance, titanium chloride, on careful
addition of water, can become ClTi(OH)3, Cl2Ti(OH)2,
ClgTi(OH), all of which are intermediate products between
the tetrachloride and the tetrahydroxide ; such compounds
are termed "basic chlorides." Anhydrous stannic chloride
is a fuming liquid, formed by the distillation of a mixture of
the metal with mercuric chloride or by heating the metal
in a stream of chlorine. Lead tetrachloride is a very
unstable liquid, formed from the tetracetate, Pb(C2H3O2)4,
by converting it into the double ammonium salt with a
62 MODERN CHEMISTRY
mixture of ammonium chloride and concentrated hydro-
chloric acid; this salt, (NH4)t>PbC]r>, is then decomposed
by concentrated sulphuric acid, when the tetrachloride
separates as a heavy liquid. It at once decomposes into
PbCl2 + C10 on warming ; hence PbO2, when warmed
with hydrochloric acid, undergoes the change: PbO,-t-
4HC1. Aq - PbCl, + Aq + C12.
Tin and lead resemble elements of the zinc group in
forming dichlorides. On dissolving tin in hydrochloric
acid the dichloride is formed ; and a solution of the tetra-
chloride, when exposed to the action of nascent hydrogen,
yields the lower chloride. This action may be thus for-
mulated :— S n C14. Aq + 2H = SnCl2. Aq + HC1. Aq.
Stannous chloride is a white, soluble salt, crystallising with
water of crystallisation. Lead dichloride, on the other
hand, is sparingly soluble in cold water ; it is formed when
a soluble lead salt, such as the nitrate, is mixed with the
solution of a chloride: Pb(NO3)2.Aq+ 2NaCl.Aq = PbCl2
+ 2NaCl. Aq. The bromide and the iodide are also
sparingly soluble, and are similarly produced.
With arsenic and the remaining members of that group
we may notice the same characters : the anhydrous chlorides
produced by the action of chlorine on the element, or,
when it is not available, on a mixture of the oxide with
carbon at a red heat ; the aqueous solution produced by
dissolving the oxide or hydroxide in hydrochloric acid.,
Basic chlorides are also known, e.g. ClAsO, CISbO, and
ClBiO, from the trichlorides ; and OPC13, and OSbCl3,,
from the pentachlorides, on reacting with a small amount
of water.
Mass=Action. — The action of mass, that is, the quan-
tity of a compound in unit volume, is well illustrated by the
action of water on antimonious chloride. A solution of this
salt in hydrochloric acid gives a precipitate on adding water:
SbCl3.nHCl.Aq + H2O = OSbCl + (n + 2)HCl.Aq.
Here the increase in the number of molecules of water
THE HALIDES 63
causes the precipitation of the basic chloride ; on adding
more hydrochloric acid, however, so as to increase its
active mass, the reaction is reversed, and the precipitate re-
dissolves : OSbCl+(n + 2)HCl.Aq = SbCl3.Aq.nHCl-h
Ht,O. Above a certain concentration of water SbOCl is
stable ; above a certain concentration of hydrogen chloride,
SbCl3.
The higher halides of molybdenum, tungsten, and
uranium, themselves prepared by the action of halogen on
the element, yield tue lower halides on heating. They are
volatile, coloured bodies, soluble in water ; the higher ones
are decomposed by water.
The elements chromium, manganese, iron, cobalt,
and nickel, although not all belonging to the same series,
show, nevertheless, a gradation of properties. The dihalides
of all are known in the dry state ; they are most readily
obtained by heating the metal in a current of hydrogen
halide, if required anhydrous ; or if in solution or crystal-
lised with water, by dissolving the oxide or carbonate in
the halogen acid and evaporating until crystallisation ensues.
As examples: Fe + 2HC1 = FeCL + H.,0 ; MnCO -f
2HBr. Aq - MnBr2. Aq + H2O + CO2.
The trihalides are best made by heating the elements,
in a current of halogen, if required anhydrous ; if in solu-
tion, by dissolving the oxide or hydroxide in the halogen
acid. The trihalides of manganese and cobalt are very
unstable ; and if the corresponding oxides be treated with
halogen acid, a portion of the halogen is evolved, thus :
Fe9O3 + 6HCl.Aq - 2FeCl3.Aq + 3HQO ; Mn.,O8 +-
6HC1. Aq - 2MnCl3Aq + sH2O. But MnCirAq gradu-
ally decomposes, especially if temperature is raised, thus :
2MnCl3.Aq=2MnCl2.Aq + Cl2. And if MnO2 be em-
ployed, chlorine is evolved from the outset : 2MnO2 -f-
8HCLAq=2MnCl3.Aq + 4H2O + CJ2; the MnCl3 de-
composing further on standing or on rise of temperature.
With Co2O3 a transient brown coloration is noticeable on.
adding hydrochloric acid, implying the momentary forma-
64 MODERN CHEMISTRY
tion of CoCl8.Aq ; but it is at once resolved into CoCl.2.Aq
and free chlorine.
Oxidation and Reduction. — As already remarked,
the raising of the valency of an element is often spoken of
as " oxidation ; " the reducing of the valency, as " reduc-
tion." The tendency of chromous halides to transform
into chromic compounds is so great, that it is not possible
to expose them to air without the change taking place,
and consequently the reduction of chromic compounds to
chromous is a difficult operation. But with iron, both
classes of compounds have nearly equal stability ; hence
oxidation and reduction play a great part in their formation.
The action of nascent hydrogen from any source reduces
ferric halide into ferrous: FeCl3.Aq + H = FeCl9.Aq +
HC1. Aq. Similarly, a ferrous halide, in presence of halogen
acid and either free or nascent oxygen, is oxidised to a
ferric : aFeCl,. Aq + 2HC1. Aq + O = 2FeCl3. Aq + H,O.
Or the halogen itself may be used to effect the change :
2FeCl2.Aq + Q2 = 2FeCJ3.Aq. On evaporating these
solutions, hydrolysis takes place partially ; thus ferric
chloride yields compounds of a basic character, such as
(OH)FeCl2, (OH)2FeCl, which are partly hydroxide,
partly chloride. This statement applies to the halides of all
these metals.
Colour of Ions. — The triad and dyad ions in the case
of these metals exhibit remarkable differences of colour.
Thus chromous ions are blue, chromic, green ; basic ferric
ions are orange-yellow, ferrous, pale green ; manganic, brown,
manganous, pale pink ; cobaltous, red, and nickelous, grass-
green. Hence a change in the ionic charge of the metallic
ion is accompanied by a striking colour-change.
The halides of the palladium and platinum groups
of metals closely resemble in character those of gold,
which have already been described. The dihalides of the
palladium group are all soluble, save PdI0, which is pre-
pared by precipitation with potassium iodide. Nitro-hydro-
chloric acid yields the higher chloride ; it remains on evapo-
HALIDES OF COMPLEX GROUPS 65
ration. These form with chlorides of the alkalies double
salts, e.g. RuCl3.2HCl, RhCl3.2HCl, and PdCl4.2HCl;
the latter are probably ionised as KK and PdCl6, &c.
Chlorine also acts directly on red-hot metals of the platinum
group, forming a mixture of chlorides ; these, on heating, lose
chlorine, giving lower chlorides. Solutions of the halides
can also be prepared by the action of the halogen acid on
the respective oxides. On heating to a high temperature,
all these halides are decomposed into the metal and halogen.
The compounds K2PtCl6 and (NH4)2PtCl6 require special
mention ; they are orange salts, nearly insoluble in water,
and are used as tests for potassium and ammonium, and also
as a precipitant in estimating these ions. Their existence is
probably to be ascribed to the power possessed by chlorine of
sometimes acting as a triad, and the structural formula is be-
KCi=Cl\ /Cl NH4C1=C1\ /Cl
lievedtobe >Pt< and >Pt< .
KC1= Cr XC1 NH4Cl=C2i/ XC1
Halides of certain complex groups are also known.
When these contain oxygen or hydroxyl, (OH), they are
generally termed basic salts or halo-acids ; they will be
considered later. The others may be divided into two
classes : those like ammonium halides, and those derived
from hydrocarbons.
Ammonium and phosphonium halides. — These hal-
ides, which are formed by direct addition of the hydrogen
halide to ammonia or to phosphine, closely resemble in
colour, in crystalline form (cubic), and in reactions, the
halides of the lithium group of metals. On mixing a solution
of ammonia and hydrochloric acid, for example, the combina-
tion occurs: NH3.Aq + HCl.Aq==NH4Cl.Aq ; and on
evaporating the solution to dryness, ammonium chloride is
left in an anhydrous state. From the conductivity of
ammonia solution, it is known to contain a certain amount of
NH4OH in an ionised condition ; and the equation may be
written: NH4OH.Aq + HCLAq = NH4Cl.Aq + H2O.
VOL. II. E
66 MODERN CHEMISTRY
As the hydroxyl ion is removed from the solution by the for-
mation of practically non-ionised water, more and more am-
monium hydroxide is formed to maintain equilibrium between
the NH3. Aq and the NH4OH. Aq ; and the whole is ulti-
mately transformed. The rate of transformation, however,
is a very rapid one. Combination has been shown not to
take place between perfectly dry ammonia and dry hydrogen
chloride ; hence it does not seem unlikely that ionisation
may occur, either in the gaseous state, or more probably on
the surface of the vessel in the condensed layer of moisture
which appears always to adhere to all solid surfaces. Once
started, combination occurs continuously until the reaction is
complete. Ordinarily "dry" ammonia, however, at once
gives a dense cloud with hydrogen chloride, bromide, or
iodide. Again, perfectly dry ammonium chloride has the
vapour-density 26.25, corresponding to the molecular
weight (N= I4 + H4 = 4 + C1 = 35.5) = 53.5 ; whereas, if
moist, the density is half that amount, corresponding with
a mixture of NH3 = 1 7 and HC1 = 36.5. These compounds
have densities of 8.5 and 18.25 respectively, and a mix-
ture in equal proportions of each has a density the mean of
the two, viz., 13.125. It appears necessary that ionisation
+
into NH4 and Cl should take place before dissociation into
NH3 and HC1 is possible. The electrically neutral body
+
NH4C1 can volatilise unchanged ; the ions NH4 and Cl
are incapable of volatilisation as such, and in volatilising
unite their opposite charges, and form the two electrically
neutral compounds HC1 and NHg.
PhospMne, PH3, also unites with hydrogen chloride,
but only under high pressure, at the ordinary temperature.
On the other hand, phosphonium iodide, PH4I, is pro-
duced by the union of phosphine with hydrogen iodide
under atmospheric pressure ; it forms white, cubical
crystals, which, like ammonium chloride, dissociate when
heated. The hydrides of arsenic and antimony form no
such compounds.
PHOSPHINE GROUPS 67
It must be assumed that these compounds are formed
with change of valency of the nitrogen or phosphorus ; the
triad becomes pentad ; the NmH3 becomes H4NVC1. On
distilling with sodium hydroxide or slaked lime, water is
formed, and the element is reduced to its original triad con-
dition, thus: NH4Cl.Aq + NaOH.Aq = NH4OH.Aq +
NaCLAq, and NH4OH.Aq = NH3 + H2O.Aq, two
electrically neutral bodies.
Carbon shows no such tendency to change valency.
The hydrocarbons of the methane series are " saturated/'
i.e. they have no tendency to take up any other element.
Hence halogen must replace hydrogen. This can be done
either directly, by the action of the halogen on the hydro-
carbon, as, for instance, CH4 + C12 = CH3C1 + HC1 ; or
indirectly, by the action of the halogen acid on the hydr-
oxide : CH3OH + HC1 = CH3C1 + H2O. Such hydr-
oxides are termed alcohols ; that derived from ethane,
C2H6, is the ordinary anhydrous alcohol of commerce ; its
formula is C9H5OH, and the corresponding chloride of
ethyl is C0H,C1. It will be remembered that the struc-
H\ /H
tural formula of ethane is H-^C — QJ-H, and that of
H/ ^H
H\ /H
ethyl chloride is H-^C— C^-C1. There is, however, a
H/ \H
difference between the formation of ethyl chloride, for
example, and of sodium chloride. Whereas sodium chloride
is ionised in solution in water, ethyl chloride is insoluble,
and is therefore non-ionised. Hence the action is a slow
one ; the alcohol is saturated with hydrogen chloride,
allowed to stand for some hours, and distilled ; ethyl
chloride, being volatile, passes over ; it is a gas, condensing
at about 12° to a mobile colourless liquid. It is probable
that the hydrogen chloride is ionised in solution in alcohol ;
the alcohol is also possibly ionised to a minute extent ;
68 MODERN CHEMISTRY
water is formed by the union of the hydrogen and hydroxyl
+
ions, and non-ionised ethyl chloride distils over : C0H.OH
+ HCl.Alc = H2O + C,H5Cl. But this suggestion, it
must be admitted, is somewhat speculative, and is based
only on analogy with reactions of a more familiar nature.
The formation of some of the halogen compounds of the
olefines, and of hydrocarbons of the acetylene and benzene
series, has already been alluded to on p. 48.
CHAPTER IV
Hydroxides and Acids — "Insoluble Substances" —
Indicators— Preparation of Basic Oxides— Pro-
perties of the Basic Oxides and Hydroxides —
Sulphides — The ' ' Solubility-Product ' ' — Basic
Oxides and Hydroxides of Complex Groups:
Alcohols, Aldehydes, Ethers; and Sulphines,
Amines and Phosphines.
The Oxides and Hydroxides, Sulphides and
Hydrosulphides, Selenides and Tellurides. —
Owing to the dyad valency of oxygen, sulphur, selenium,
and tellurium, compounds of these elements are more
numerous than those of the halogens. And whereas double
halides of hydrogen and other elements are not numerous,
being confined to such bodies as H2SiF6, HBF4, H.7PtCl6,
and a few others, those of the oxides are very numerous,
and form two important classes, the " hydroxides " and
the "acids."
Hydroxides and Acids. — Members of both these
classes may be regarded as hydroyxl, that is, water minus
one atom of hydrogen, -OH, in combination with elements,
but they differ radically in that the true hydroxides ionise
+ - ++ -
into element and hydroxl, thus: NaOH.Aq, Ca(OH)9.Aq,
+ + + -
Bi (OH)3; whereas acids ionise into hydrogen and a
+ • + -
negatively charged radical, thus : HOCl.Aq, HNO3.Aq,
+ - - + -
H(HSOJ.Aq, H0SO4.Aq, and many others. There
69
70 MODERN CHEMISTRY
are certain hydroxides in which the ionisation may take
either form ; such compounds are said to be either "basic "
or " acid " according to circumstances ; thus, aluminium
hydroxide, A1(OH)3, is basic; with hydrochloric acid
+ + + -
it reacts in the following manner: Al(OH)3.Aq +
3HCl.Aq = Al Cl3.Aq + 3H2O; on the other hand, when
caustic soda is presented to aluminium hydroxide, it forms
sodium aluminate, NaAlO2.Aq, a derivative of the acid
HAlOi;.Aq, which is formed from A1(OH)3 by loss of
water : "A1(OH)3 = O=A1OH + H0O. The ions in the
+
latter case are H and A1O9, and the reaction takes place
between HA1O2 and NaOH, thus: HAlO2.Aq +
NaOH.Aq = NaAl62.Aq + H2O. It is generally the
case that the acids are derived from hydroxides which
have lost a portion of their hydrogen as water. They
are, like O=A1OH, partly oxide, partly hydroxide.
"Insoluble Substances." — The hydroxides are,
with some exceptions, generally spoken of as insoluble in
water. The word " soluble " is a relative term ; it is
probable that very few, if any, substances are absolutely
insoluble. Silver chloride is usually regarded as wholly
insoluble in water, but pure water shaken up with that salt
acquires increased conductivity, showing that some chloride
must have gone into solution. In one of the equations
given above, A1(OH)3 is followed by "Aq," implying
that it is dissolved and ionised in solution. This method
of writing is perfectly correct for the portion which is dis-
solved, but that constitutes only a very minute fraction of
the whole. What is dissolved, however, is ionised and
enters into reaction, and when it has been removed, as in
the equation given, with formation of practically non-ionised
water-molecules, its place is taken by more : equilibrium
tends to establish itself between the dissolved portion and
HYDROXIDES AND ACIDS 71
the portion remaining undissolved. We know well that if
excess of common salt be placed at the bottom of a vessel
of water it will not all dissolve, but, as the dissolved portion
diffuses away into the upper layers of water, its place is
taken by fresh salt, which dissolves, until, if sufficient time
be given, the whole solution becomes saturated with salt.
Similarly, the removal of the aluminium — as ions, it may
be — and of the hydroxyl of the aluminium hydroxide,
A1(OH)3, as water, on treatment with an acid, causes a
fresh portion of the hydroxide to go into solution, and this
continues to go on until all has undergone reaction.
The hydroxides of the elements may be classified like
the halides ; the analogy between the formulae is seen on
comparing the tables on pp. 72, 73, with those on p. 50.
Oxygen compounds of fluorine are wanting.
[C1(OH)7], [C1(OH)5], [C1(OH)3], Cl(OH).
[OC1(OH)5], [OC1(OH)3], OCl(OH), C12O.
[02C1(OH)3]> 02C1(OH), [C1203].
03C1(OH), [C1205].
[C1207].
The formulae enclosed in brackets are of unknown sub-
stances. The whole scheme is given in order to show the
gradual loss of water of an ideal heptoxide.
The compounds I(OH)6(ONa), OI(OH)5, O2I(OAg)3,
and O3I(OAg) are known, corresponding to the theoretical
perhalic acids. Those corresponding to the halic acids are
O2Br ( OH ) bromic acid, and I ( OH ) 5 and O2I (OH ) , iodic
acids. Br(ONa) and I(ONa), named respectively hypo-
bromite and hypoiodite of sodium, are also known.
It may be noticed that the formulae of some of the com-
pounds of chromium are analogous to those of sulphur and
of molybdenum ; other compounds, on the contrary, show
more resemblance to those of iron. While manganese, like
chromium, also shows analogy with iron, it too forms
O2Mn(OK)2, like O2S(OH)2 or O 2Mo( OH )2, termed
potassium manganate, as the others are hydrogen sulphate
and hydrogen molybdate ; and also MnO0, analogous to
MODERN CHEMISTRY
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O tn O 'Oj2O 'O 'O ' 'OO
^•^ in' "o'c/^ r? TT v? ~^r.£*
W W fflffl
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OXIDES AND HYDROXIDES 73
9, °? Q
I-IH HH , -*
c ^ : : : : *
O* O 9 § o* O
u u ^ ^ £> D
1 . i „ ^ 1 1 . i ' I
^ 9^o ^ ^ o 5- ^
duo^ oo^oo
as * « •: «
o 9. » d5 c? ' c? 6s
>* > 9 j? ^* rt cT
O O > Z X H H
£
1 1
O OH
I : o : o o:
O ^ « OH
£ : : £
o o o o o • o
C C *O T3 T3 bO
N N U O O X
° : : O q,
3 bO tuO
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74 MODERN CHEMISTRY
SO2 and MoO2 ; but manganese also forms O3Mn(OK),
termed potassium permanganate, which is analogous in formula
as well as in crystalline form with potassium perchlorate,
O3C1(OK). It is convenient, however, also to include
chromium and manganese in the iron group of elements.
Cr(OH)3 Mn(OH)8 Fe(OH)3 Ni(OH)3 Co(OH)3
OCr(OH) OMn(OH) OFe(OH)
Cr2O3 Mn2O3 Fe2O3 Ni2O3 Co2O3
Cr(OH)2 Mn(OH)2 Fe(OH)o Ni(OH)2 Co(OH)2
CrO MnO FeO NiO CoO
Elements of the palladium group have a very wide range
of valency ; hence they form a large group of compounds.
OsO4 IrO3
Rh03 02Ru(OK)2 ... 02Os(OH)2 CX>Ir(OK)2
Rh(OH)4 Ru(OH)4 Pd(OH)4 Os(OH)4 Ir(OH)4 Pt(OH)4
Rh02 Ru02 Pd02 Os02 Ir02 PtO2
Rh(OH)3 Ru(OH)3
Rh0O3 Ru2O3 ... OsoO3 Ir2O3
Pd(OH)2 ... ... Pt(OH)2
RhO RuO PdO OsO IrO PtO
The hydroxides of lithium, sodium, potassium,
rubidium, and caesium are all soluble white compounds,
melting to colourless liquids at a red heat. They do not
lose water, even at the highest temperatures, hence the
oxides cannot be prepared from them ; indeed, the oxides
are produced only by the action of the metal on the hydr-
oxide, at a high temperature ; for instance, 2NaOH +
2Na = Na2O + H2. They are white, non-crystalline sub-
stances, combining at once with water to form the hydr-
oxides Na2O + H2O = 2NaOH. The hydroxides are
prepared from the carbonates by boiling a solution with
slaked lime (calcium hydroxide) : Na2CO3.Aq +
Ca(OH)2.Aq = 2NaOH.Aq + CaCO3; or by heating to
redness a mixture of the carbonate with ferric oxide, when
the ferrite is formed : K2CO3 + Fe2O3 - 2KFeO2 + CO2.
On treatment with water, potassium ferrite is decomposed,
thus: KFeO2 + 2H2O.Aq = KOH.Aq + Fe(OH)3.
INDICATORS 75
In either case, the solution of hydroxide is evaporated to
dryness in an iron vessel and fused.
These hydroxides are said to be basic, for they are
neutralised by acids, forming salts. Thus, with hydro-
chloric acid, KOH.Aq + HCl.Aq = KCl.Aq + H20,
the point of neutralisation — that is, when the acid and base
are present in theoretical quantity to form the salt and water
— is determined by the addition of an " indicator."
Indicators. — The most important indicators are litmus,
phenol-phthalein, and methyl-orange. Litmus is, a weak
acid, red in colour, the salts of which are blue. When
dissolved in water, the molecule is hardly at all ionised,
hence the red colour of the acid is alone visible. If a base
such as sodium hydroxide is added, which, in aqueous solu-
+
tion, is largely ionised into Na.Aq and OH.Aq, the
hydroxyl ions combine with that portion of the hydrogen
ions of the litmus acid which exist in solution ; when these
are withdrawn, more hydrogen ions take their place, and the
solution acquires the colour of the ion of the litmus acid,
viz., blue. Conversely, if an acid be added to a base in
which the blue litmus ions are present, the hydrogen ions of
the acid combine with the hydroxyl ions of the base, forming
water, so long as any are present ; after they are all in
combination they convert the ion of the litmus acid into the
red acid, non-ionised, and there is a marked colour-change.
As the colours of the litmus acid and of its ion are both
very bright, the presence of a mere trace of the indicator
suffices. Phenol-phthalein, like litmus, is also a weak acid,
that is, it is hardly ionised at all in dilute solution ; the acid
is colourless, but the ions are pink, hence the addition of a
trace of free alkali causes the colourless solution to become
pink. But this indicator gives results only with strong bases,
like the hydroxides of the alkalies ; with ammonium hydr-
oxide, present in a solution of ammonia in water, it is not a
good indicator, for NH4OH is too weak a base, i.e. the
hydroxyl and ammonium ions are present in too small amount
76 MODERN CHEMISTRY
to liberate the ions of the phenol-phthalei'n, unless much
ammonium hydroxide is present in solution. Hence the
presence of a trace of free ammonium hydroxide is not
revealed by that indicator. Phenol-phthalei'n is therefore
serviceable only with strong bases, but it may be used for
weak acids. Methyl-orange, on the other hand, is a com-
paratively strong acid ; with a weak base it forms the ions
of a salt, and it may therefore be used for weak bases like
ammonium hydroxide, or for strong bases like the hydrox-
ides of the alkali metals ; but it is too strong an acid to
serve well as an indicator of excess of a weak acid, such as
carbonic or acetic acid. Its colour-change is from orange
to orange-pink.
Preparation of Basic Oxides. — The hydroxides of
the metals of the sodium group, as already mentioned, do
not lose water on heating, and the oxides, therefore, cannot
be thus obtained. Neither do their carbonates lose carbon
dioxide, nor their nitrates oxides of nitrogen, save at an im-
practicable temperature. But all other basic oxides may be
prepared by heating the hydroxides, carbonates, or nitrates
of the metals, and a few may be obtained by heating the
sulphates. Calcium and strontium oxides are generally
prepared from the carbonates, which are found as minerals,
named limestone and strontianite respectively. The opera-
tion of preparing " quicklime " or calcium oxide is techni-
cally, but wrongly, called " burning." Alternate layers of
lime and coal are placed in a tower of brick or stone,
termed a limekiln ; the coal is set on fire, and its heat
expels the carbon dioxide from the carbonate : CaCO3 =
CaO+CO9. If calcium carbonate be heated in a closed
vessel, however, so that the carbon dioxide does not escape,
the dissociation proceeds until the amount of carbon dioxide
in the vessel has reached a certain proportion, which is per-
fectly definite for each temperature, or until the carbon
dioxide has attained a certain " concentration." The reaction
then stops. But if the carbon dioxide be removed as it is
formed, the reaction goes on to the end, until all carbon
BASIC OXIDES 77
dioxide has escaped. The draught in the kiln removes the
carbon dioxide, hence the product is calcium oxide. Stron-
tium carbonate is causticised in the same way as limestone ;
but the temperature for witherite (BaCO3) is inconveniently
high ; baryta is consequently prepared by heating the nitrate,
Ba(NOg),, ; it may be supposed to split into BaO and
N.,O5 ; the latter, however, decomposes even at moderate
temperatures into NO2 and O ; hence the equation is :
2Ba(NO3),= 2BaO + 4NO2 + O2. These oxides are
whitish-grey solids, volatile at the temperature of the elec-
tric arc, and combining with water with great rise of tem-
perature to form hydroxides. The hydroxides are sol-
uble in water — barium hydroxide most, calcium least. An
aqueous solution of the former deposits crystals of a hydrate,
Ba(OH)2.8H2O.
The sparing solubility of calcium hydroxide makes it
possible to precipitate it by the addition of caustic alkali to a
soluble salt of calcium, provided too much water is not pre-
sent : CaCl2.Aq + 2NaOH.Aq = Ca (OH), + aNaCl.Aq.
Of course, a saturated solution of calcium hydroxide remains,
hence the precipitation is not complete. This plan is appli-
cable to the preparation of all hydroxides which are insoluble
in water, unless they dissolve in excess of the caustic alkali ;
if they do, they are said to display " acid " properties.
Beryllium and magnesium hydroxides are thus precipitated :
MgCl2.Aq + 2KOH.Aq = Mg(OH)2+2KCl.Aq. The
hydroxide may be filtered off and dried, and the white mass,
on ignition, leaves the oxide as a white powder. By this
means, too, the hydroxides of copper (cupric, Cu(OH)2),
silver, AgOH, zinc, Zn(OH)2, cadmium, Cd(OH),,
aluminium, A1(OH)3, scandium, yttrium, lanthanum.
and ytterbium, gallium, indium, and thallium, with
similar formulae, titanium, zirconium, thorium, with
formulas OM(OH)2, (where M stands for an element of
that group; germanium, tin (stannous, Sn(OH)2, and
stannic, SnO2.nH,O), lead (plumbous, Pb(OH)2), bis-
muthous, Bi(OH)3, chromic, Cr(OH)3, and chfomous,
78 MODERN CHEMISTRY
Cr(OH)2, manganic and manganous, ferric and ferrous,
cobaltous and nickelous : in short, from all elements which
form " basic " hydroxides. And from almost all these the
oxides may be obtained by heating the hydrates to redness.
Excess of the precipitant, however, must be avoided in many
cases, for some of these hydroxides display acid properties
if in presence of excess of alkali. Thus, for example, if
excess of sodium hydroxide or potassium hydroxide be
added to the solution of a soluble salt of zinc, such as the
chloride, nitrate, or sulphate, the first change, as already
shown, is the precipitation of the hydroxide ; but on additi :>n
of excess of alkali, the precipitate redissolves, forming the
+ +
compound Zn(OK)9.Aq, of which the ions are K, K, and
ZnO9 ; this compound is thrown down by alcohol, in which
it is insoluble. It is generally named zincate of potassium.
Cadmium forms a similar compound, but that of aluminium
has the formula OAl(OK) ; the hydroxide, A1(OH)3, on
losing water is transformed into the condensed hydroxide,
OAl(OH), which may be termed aluminic acid, of which
the hydrogen atom is exchangeable for metals. Stannous
and plumbous hydroxides dissolve in excess of alkali, doubt-
less forming compounds similar to that of zinc ; and chromic
hydroxide is soluble in cold solution of caustic alkali, form-
ing, no doubt, a compound analogous to that of aluminium ;
but it is decomposed on warming, with reprecipitation of the
hydroxide. The hydroxides of all these elements may also
be precipitated by a solution of ammonium hydroxide, and
some of them are redissolved ; but the compounds formed
are of a different nature from those described in the case of
zinc, &c., and will be afterwards considered.
Properties of the Hydroxides. — As regards the
properties of the hydroxides, that of copper (cupric) is light
blue, and of silver, brown ; chromic hydroxide is grey-
green, and chromous, yellowish ; manganic, brown, and
manganous, very pale pink ; ferric, rust brown, and ferrous,
white when pure, but usually dirty green ; cobaltous, dingy
PROPERTIES OF HYDROXIDES 79
red, and nickelous, grass-green. The others are all white
amorphous bodies, and they all yield oxides on heating.
Cupric oxide is black ; even when boiled with water the
hydroxide loses water and changes colour. Argentous
oxide is brown ; when heated to redness it loses oxygen,
leaving a residue of metallic silver. Zinc oxide is yellow
when hot and white when cold ; cadmium oxide is a
brown powder ; the oxides of aluminium, scandium,
yttrium, lanthanum, ytterbium, gallium, and indium, and of
titanium, zirconium, thorium, germanium, and stannic oxide
are white powders ; thallium oxide is a yellow powder ; tin
monoxide is a black powder ; that of lead (litharge, massi-
cot) is a yellow substance, fusible at a red heat ; bismuth
sesquioxide is a yellowish powder ; chromic, ferric, and
manganic are respectively green, rust-red, and brown ;
chromous oxide is unknown, for any attempt to dry it
results in the decomposition of water, the absorption of its
oxygen by the chromous oxide which becomes chromic
oxide, and the evolution of hydrogen. Ferrous hydroxide
can be dried, but only with rigid exclusion of air ; it
is a black powder. Manganous oxide is greyish-green,
cobaltous, olive-green, and nickelous also greyish-green.
Manganous hydroxide must also be dried in absence
of air.
These hydroxides and oxides are named bases. There
are some basic oxides, which are precipitated by adding a
hydroxide, such as that of sodium, to a soluble salt, and to
which there is no corresponding hydroxide. This is the
case with the oxides of mercury. On referring to the
table of halides on p. 50, it will be seen that the chloride
of mercury has the formula HgCl9. This compound,
commonly called corrosive sublimate, when treated in solu-
tion with sodium hydroxide, gives a precipitate, not of
hydroxide, as might be expected, but of oxide : HgCl0. Aq
+ 2NaOH.Aq = HgO + 2NaCl.Aq + H2O. Similarly,
a soluble mercurous salt, such as mercurous nitrate,
Hg0(NO3)0, on treatment with an alkali gives a precipitate
8o MODERN CHEMISTRY
of mercurous oxide: Hg2(NO3)0.Aq + 2NaOH.Aq =
Hg2O + 2NaNO3.Aq + H26. These are cases of relative
stability ; for, as has been already remarked, on boiling a
solution from which cupric hydroxide has been precipitated,
the blue hydroxide is changed into black oxide ; other
hydroxides lose their water at a still higher temperature ;
while those of the alkaline metals may be volatilised without
decomposing.
Oxides produced by Heating Carbonates.—
Most of the basic oxides may also be prepared by heating
the carbonates, a class of salts afterwards to be discussed.
The carbonates of the alkali metals, however, are not thus
decomposed ; like their hydroxides, they may be volatilised
without decomposition. But all other carbonates are de-
composed by exposure to a red heat. The process has
already been described as a method of manufacturing quick-
lime. Most carbonates, however, do not require the same
high temperature ; a dull red heat suffices. And the
oxides do not, as a rule, recombine with the carbon dioxide
expelled, as does lime ; hence there is no danger of re-car-
bonating the oxide.
Oxides produced by Heating Nitrates. — The
nitrates, too, of nearly all the basic metals, yield the respec-
tive oxides when they are heated to bright redness. The
nitrates of the alkali metals in this instance, as in others, do
not behave in this way. When heated they lose oxygen,
but only at a very high temperature, forming the nitrites, a
class of salts afterwards to be described. Thus, potassium
nitrate undergoes the decomposition : 2KNO3 = 2KNO9 +
O2. The product of heating the other nitrates, however, is
the oxide, while a mixture of oxides of nitrogen is evolved.
This may be supposed to take place in two stages : first,
the nitrate may be imagined to decompose into the oxide
and nitrogen pentoxide, thus : Zn(NO8)2 = ZnO + N2O5 ;
the last compound is very easily decomposed by heat, and
yields a lower oxide of nitrogen : 2N2O. = 4NO2 + O2 ;
while if the temperature is over 600°, which is usually
SULPHIDES AND HYDROSULPHIDES 81
exceeded in decomposing the nitrates, the nitric peroxide is
partly further decomposed into nitric oxide and oxygen :
2NO2 = 2NO + O2. The products, therefore, are NO2,
NO, and O2.
A metal which forms two oxides, one containing more
oxygen than the other, if the nitrate of the lower oxide is
heated, yields the higher oxide. Cases of this are mercury,
tin, and iron. Mercurous nitrate, carefully heated, gives,
not mercurous oxide, Hg9O, but mercuric oxide, HgO :
HgNO3 - HgO + NO2 ; similarly Sn(NO3)2 yields SnO2,
and not SnO ; and Fe(NO3)2, Fe2O3, and not FeO.
Oxides produced by Heating Sulphates. —
The sulphates require a higher temperature than the nitrates
for their decomposition, consequently they are not generally
used as a source of oxides. But the equivalents of mag-
nesium, zinc, and some other metals have been determined
by estimating the weight of oxide obtainable on heating a
weighed amount of sulphate ; and ferrous sulphate has been
distilled in fireclay retorts for many years past at Nord-
hausen, in Saxony, for the purpose of making " Nordhausen
sulphuric acid," H2S2OK, and red oxide of iron, Fe2O3,
which, made in this way, has a fine colour, and is used as a
paint. When a sulphate is heated, the gas which escapes
is not entirely SO3, as might be imagined from the
equation : MgSO4 = MgO + SO3 ; the high temperature
decomposes most of the sulphur trioxide into the dioxide,
SO0, and oxygen ; and the oxygen, in the case of ferrous
sulphate, oxidises the FeO into Fe2O3.
Sulphides and Hydrosulphides. — The analogy
between the elements oxygen and sulphur is well shown
by comparing the sulphides of the elements of which the
oxides have been described. Elements of the lithium
group form both hydrosulphides and sulphides ; thus we
know sodium hydro sulphide, NaSH, analogous to the
hydroxide NaOH, and sulphide, similar in formula to
the oxide Na9O, Na9S. Hydrogen sulphide is a
weak acid ; hence, on passing hydrogen sulphide through
82 MODERN CHEMISTRY
a concentrated solution of sodium hydroxide at 95° until
saturation is complete, white crystals of NaSH.2H2O
+ -
deposit on evaporation. The equation is : NaOH.Aq
+ HSH.Aq = NaSH.Aq + H,O. On mixing the solu-
tion with an equivalent quantity of sodium hydroxide and
+
evaporating, the sulphide is produced : Na SH . Aq +
+ - + - -
NaOH.Aq = Na2S.Aq + H2O. Here it must be sup-
posed that the hydrogen of the hydrosulphide is present as
an anion, and that it reacts with the hydroxyl of the caustic
soda, forming water, while the sodium sulphide remains in
solution in an ionised form, and can be recovered on
evaporation in crystals with 9H2O. Similar compounds
exist with potassium.
Calcium, strontium, and barium also form hydro-
sulphides and sulphides, analogous in formula to the
hydroxides and oxides. They are similarly prepared to
the sodium compounds, but, as the metals are dyads, their
formulas are M(SH)2 and MS; and there is an inter-
mediate compound between the hydroxide and hydro-
sulphide, having, in the case of calcium, the formula
HSCaOH. They are also soluble in water. Magnesium,
too, forms a hydrosulphide, probably Mg(SH)2; it is
prepared by passing sulphuretted hydrogen into water in
which magnesium oxide is suspended. It is unlike the
hydrosulphides of the alkalies, for while they do not
decompose with water, it, on the contrary, when its solution
is heated to 80°, reacts with water, yielding hydroxide
and sulphuretted hydrogen: Mg(SH)2.Aq + 2HOH =
Mg(OH)2 -f 2H2S. The probable explanation of this
change is that water is not wholly non-ionised, but that
there are present some hydrogen ions ; these are not so
inconsiderable in number, compared with those of the weak
acid H2S ; on raising temperature, a certain amount of
hydrogen sulphide is liberated, and, being volatile, it escapes,
THE SOLUBILITY-PRODUCT 83
and is no longer present to act on the magnesium hydroxide
and reconvert it into sulphide.
Sulphides of boron, aluminium, chromium, and
silicon are at once decomposed by water, and cannot,
therefore, be produced in aqueous solution. They are
white substances formed by heating the elements to a high
temperature in a current of sulphur vapour.
The sulphides of copper, silver, gold, cadmium,
mercury, indium, thallium, tin, lead, arsenic, anti-
mony, and bismuth, and of the metals of the palladium
and platinum groups, are all insoluble in water, or, to be
more accurate, very sparingly soluble. They form no
hydrosulphides. Hence they are precipitated from soluble
salts of these metals by addition of sulphuretted hydrogen ;
they form flocculent precipitates, usually characterised
by striking colours, and are therefore generally used as a
means of recognising the metal. CuS, Ag9S, Au0S3,
HgS, T12S, T12S3, PbS, PtS2, and the other sulphides
of the platinum group of metals are black ; CdS, SnS2, and
As.,S3 are yellow ; ln9S3, SnS, and Bi9S3 are brown, and
Sb9S3 is orange. These sulphides are not attacked by dilute
acids. On the other hand, the sulphides of zinc, manganese,
iron, cobalt, and nickel are not precipitated by hydrogen
sulphide, but they are thrown down by a soluble sulphide
or hydrosulphide, such as those of ammonium or sodium.
They, too, form flocculent precipitates ; ZnS is white, MnS
pink, and FeS, CoS, and NiS are black. The reason of
the difference in the behaviour of the two classes of sulphides
is an interesting one, and will be now explained.
Solubility** Product. — It has already been mentioned
on p. 14 that the rate of chemical change depends on the
amount of each of the reacting substances present in unit
volume. This last is generally termed the " concentration "
of these substances, for the more concentrated the solution
the greater the mass present in unit volume. Now, if two
+
kinds of ions, such as Na and Cl, are present in solution,
84 MODERN CHEMISTRY
necessarily in equal numbers, the solution will also contain
a certain number of molecules of non-ionised NaCl, formed
by their union, and the relative number of ions and mole-
cules will depend on the concentration ; the number of ions
in proportion to the number of non-ionised molecules will
be greater, the greater the dilution. For each dilutk
(and for each temperature) a state of balance will result;
the position of this equilibrium will depend on the relative
rate at which ionisation and union of ions to form mole-
cules go on ; if ionisation takes place twice as quickly as
combinations of ions to form molecules, then two-thirds
of the dissolved substance will exist as ions, the remain-
ing third being non-ionised molecules. If the solution is
made more concentrated by evaporation, the conditions are
changed, and the rate of ionisation is reduced compared
with the rate of union of ions with each other. Suppose
that concentration be pushed so far that solid salt separates
out ; the limit of concentration will be reached, since it is
now impossible to alter the number of ions and of molecules
in unit volume of the solution. The ratio will now remain
constant, and if c and c be the concentrations of the ions
(and they are, of course, equal), and if C be that of the
non-ionised molecules, then c.c = LC9 k being a factor ex-
pressing the relative proportions of the non-ionised mole-
cules. If k is very small, then there are many molecules
and few ions present ; if, on the contrary, k is large, the
ions are numerous and the molecules few. The expression
LC is termed the "solubility-product."
To take a specific case : — A solution of ammonia in
water consists partly of the ions NH4 and OH, and
partly of non-ionised molecules of NH4OH ; it is a
weak base — that is, the number of non-ionised molecules
is much greater than that of the ions. In a solution con-
taining 1.7 grams of ammonia per litre (one- tenth normal
solution), only 1.5 per cent, of the total number of mole-
cules exist in the ionic state. Hence a solution of ammonia,
unlike one of caustic soda or potash, gives no precipitate
INSOLUBILITY OF SULPHIDES 85
of hydroxide when added to a solution of salts of the
relatively strong bases, such as calcium, strontium, or
barium chlorides. With salts of the weaker base magnesia,
however, ammonia produces a precipitate of magnesium
hydroxide. It is possible still further to reduce the ion-
isation of ammonia solution ; this can be done by the
addition of an ammonium salt, such as the chloride, which,
like most such salts, is highly ionised. The reason is, that
+
while (concentration of NH4) x (concentration of OH) =
k x (concentration of NH4OH), if more ammonium ions be
added, the number of hydroxl ions will diminish by union
with NH4 ions, forming non-ionised ammonium hydroxide,
because the increase of the number of ammonium ions will
increase the value of the product on the left-hand side of the
equation, and in order that it may balance that of the right,
the relative number of molecules of NH4OH must be in-
creased ; and we may see that if ammonium chloride is added
to a solution of magnesium chloride, ammonia solution will
no longer produce a precipitate of magnesium hydroxide ;
the ammonia is too weak a base, that is, it contains too few
hydroxyl ions, which are the reason of its basic nature.
Let us now return to the consideration of the insolubility of
sulphides of the copper group in acids and the solubility of such
sulphides as that of zinc. No substance, as has been before
remarked, is wholly insoluble in water ; zinc sulphide, how-
ever, belongs to the very sparingly soluble compounds.
+ +
Hence the product r(Zn) xr'(S) has a very small value,
for it is equal to >£.£(ZnS), which must necessarily be very
small, seeing that the compound is so sparingly soluble.
+ +
Now, the ions of H2S are H, H, and S ; but though the
ionisation is very small, hydrogen sulphide being a very
weak acid, they are yet sufficient to reach the value of
the very small solubility-product ^.C'(ZnS). If, however,
the concentration of the S-ions is still further diminished
86 MODERN CHEMISTRY
+
by addition of some compound rich in H-ions, such as
+ - + +
HCl.Aq, then the product r(Zn) xc( S ) will be less
than >£.£(ZnS), and there will be no precipitate ; or if
hydrochloric acid be added to precipitated zinc sulphide,
it will be dissolved. On the other hand, the addition of
acetic acid, a weak acid, and poor in hydrogen ions, does
not bring about solution of zinc sulphide ; indeed, the pre-
cipitation of zinc from a solution of its acetate by hydrogen
sulphide is almost complete.
The solubility-product of copper sulphide, and of the
other sulphides which are not soluble in dilute acids, is
still less ; hence hydrogen sulphide precipitates them from
acid solution, for the concentration of the S-ions of the
hydrogen sulphide may be very much diminished without
+ +
the product r(Cu) x c ( S ) becoming less than ^.£(CuS),
for CuS is still less soluble in water than ZnS.
Oxides and Hydroxides of Complex Groups.
— The oxides and hydroxides of complex groups show
analogy in their formulae, and often in their methods of
preparation with the basic oxides and hydroxides. A few
instances of these will now be given.
Ammonia (see p. 42) is very soluble in water ; at the
ordinary temperature, no less than 800 volumes of the gas
dissolve in one volume of water, forming a very pungently
smelling solution named liquor ammonia. This solution con-
sists for the most part of a mixture of liquid ammonia with
water ; it probably also contains ammonium hydroxide,
NH4OH, and, as already mentioned, less than 1.5 per cent.
+
of the ions NH4 and OH. It is, therefore, a weak base.
Hydrazine, N2H4, also forms a hydrate, N2H5OH,
a fuming liquid with slight smell (and consequently in all
probability fairly highly ionised) ; it boils at 119°, and is
very corrosive, attacking wood, cork, and even glass. It has
a strong reducing action, so that if added to a solution of
ALCOHOLS 87
cupric sulphate which contains cupric ions, Cu, it gives an
immediate precipitate of cuprous oxide, Cu2O, nitrogen
being evolved. Like ammonia, it precipitates such hydr-
oxides as that of aluminium, iron, &c. Hydroxylamine,
NH0OH, is a somewhat similar body, produced by passing
nitric oxide, NO (see p. 97), through a mixture of granu-
lated tin and hydrochloric acid, to which a little platinic
chloride has been added ; the nascent hydrogen reduces
the nitric oxide to hydroxylamine ; it unites with the
hydrochloric acid, forming hydroxylamine hydrochloride,
NH3OHC1. After removal of the tin by addition of
sodium hydroxide and filtration, the solution is evaporated
to dryness and mixed with alcohol ; hydroxylamine hydro-
chloride dissolves, while sodium chloride remains. A solution
of the base may be obtained by addition of silver hydroxide :
NHgOHCl. Aq + AgOH. Aq = AgCl + NH3OH. Aq. If
sodium methoxide (see p. 88) be added to a solution of the
hydrochloride in methyl alcohol, the base is liberated, and
can be separated from the alcohol by fractional distillation ;
it is a volatile white solid. This compound is interesting,
because the OH group is under no circumstances an ion ; its
solution in water must contain ions of NH3OH and OH,
since it reacts like ammonium hydroxide.
Alcohols. — The hydroxides of the hydrocarbon
radicles are, as mentioned on p. 67, termed alcohols.1
Of these there are very many, but a few only will be
chosen to serve as examples : methyl alcohol, CHgOH,
ethyl alcohol, CH3— CHQOH, as types of monohydric
alcohols, which may be taken as the analogues of the
CH2OH
hydroxides of the monad metals ; glycol, , a
CH2OH
dihydric alcohol, may be likened to barium hydroxide,
1 A special class of such hydroxides derived from benzene, C6H6,
are termed phenols. "Carbolic acid," C6H3OH, is the best known
of these.
88 MODERN CHEMISTRY
CH2OH
Ba(OH)9; and glycerine (glycerol), CHOH , is a
CH2OH
trihydric alcohol, as aluminium hydroxide is a trihydroxide.
These substances differ from the hydroxides, however, by
their being non-electrolytes, and therefore non-ionised.
Or perhaps it is more correct to say that their conductivity
is of the same order of magnitude, but less in value, than
that of pure water. The corresponding halides, for ex-
ample, CH3C1, C2H4C12, and C3H5C13, are also regarded
as non-ionised ; they are practically insoluble in water.
Nevertheless, methyl chloride has been transformed into
methyl alcohol by heating with water to a high temperature
in a sealed tube under pressure — CH3C1 + HOH =
CH3OH + HC1 ; and the others, but preferably the bro-
mides, may be similarly changed into hydroxides by heating
with silver hydroxide, or with silver oxide and water :
CH2Br CH2OH
CHBr + sAgOH.Aq = CHOH .Aq + 3 AgBr. Is it
CH2Br CH2OH
possible that at a higher temperature the ionisation is suffi-
cient (though it must be exceedingly small) to produce the
interaction ?
The metals sodium and potassium dissolve in the alcohols,
with evolution of hydrogen, forming compounds somewhat
analogous to the hydroxides ; instead of hydrogen, however,
they contain a hydrocarbon group : sodium methoxide,
for example, has the formula Na(OCH3). Such sub-
stances are white solids, like caustic soda.
Aldehydes. — The alcohols, if oxidised by boiling
them with chromic acid, yield a class of bodies analogous
to the oxides, termed aldehydes : CH3— CH2— OH + O
= (CH3 - CH)"O + H2O. It will be noticed that ethane,
CH3 — CH3, has lost two hydrogen atoms, and that the
residue, CH3 — CH0, is now a dyad group, capable of com-
bination with an atom of dyad oxygen. The aldehydes are
volatile liquids, with strong odour, and those containing few
AMINES AND PHOSPHINES 89
atoms of carbon are miscible with water. They form easily
decomposable compounds with water, which are di-hydr-
oxides ; e.g. ordinary aldehyde forms CH3
OH
they are called aldehydrols. When brought into contact
with solutions from which hydrogen is being evolved, the
aldehydrols lose oxygen, and are converted into alcohols :
/OH
CH3CH/ + 2H = CH3-CH2OH + H2O.
The alcohols cannot be termed basic substances ; still, it
is evident that they show analogy with the true bases in
many respects.
Amines and Phosphines. — Derivatives of nitrogen,
phosphorus, sulphur, and even of iodine and of oxygen,
containing hydrocarbon groups, are however known,
which are true bases, though weak ones. If ammonia
in alcoholic solution be heated with excess of methyl
iodide, tetra- methyl -ammonium iodide is formed :
NH3 + 4CH3I = N(CH3)4I + sHI. This iodide, digested
with water and silver hydroxide, exchanges iodine for
hydroxyl, and after removal of the silver iodide by filtra-
tion the solution may be evaporated to dryness. The
residue is a white solid, of the formula N(CH^)4OH ;
it is termed tetra -methyl- ammonium - hy dioxide ; in
its reactions it shows great analogy with caustic potash,
having a caustic taste, and producing precipitates with
the usual salts of the metals. In solution it is more
ionised than ammonium hydroxide, though less than that
of potassium.
Phosphine, as remarked on p. 66, combines with
hydrogen iodide, forming a salt, PH4I, phosphonium iodide,
resembling ammonium chloride. But as it is decomposed
by water into phosphine, PH3, and hydrogen iodide, an
attempt to convert it into phosphonium hydroxide, PH4OH,
cannot be made. Substituted phosphonium compounds,
90 MODERN CHEMISTRY
however, are known, in which a hydrocarbon radicle, such
as methyl, replaces hydrogen. Sodium and phosphorus
combine when heated together under an oil called xylene,
forming PNa3 ; this body, treated with methyl iodide, yields
trimethyl phosphine, P(CH3)., ; with more methyl iodide
P(CH3)4I is formed; and its solution in water, which is
not decomposed by the solvent, yields with silver hydroxide
tetra-methyl-phosphonium hydroxide, P(CH3)4OH, a
base resembling the corresponding ammonium compound.
These compounds exist owing to the double valency of
nitrogen and of phosphorus, which can function either as
triad or pentad. Double valency is to be noticed also with
oxygen and with sulphur, although with the former tetrad
combinations are far from stable, while with the latter both
dyad and tetrad compounds can be formed.
Ethers. — Oxide of methyl and oxide of ethyl, which
are usually named methyl and ethyl ethers, are formed by
mixing solutions in alcohol of methyl or ethyl iodide with
sodium methoxide or ethoxide : CH3I.Alc + NaOCHg. Ale
= Nal + HgCOCHg. Ale. The ether has a low boiling-
point, and can be separated by fractional distillation from
the alcohol in which it is dissolved. Methyl ether is a
gas; ethyl ether a volatile liquid, boiling at 37°. Such
compounds can also be prepared more readily by distilling a
mixture of the alcohol with sulphuric acid, which yields
HCH^SO4, hydrogen methyl sulphate, with the alcohol :
HCH3SO4 + CH3OH = HgCOCHg + H2SO4. Now,
methyl ether and hydrochloric acid combine at a low
CH3\ /H
temperature, yielding /O\ > but it is impossible to
CH/ NCI
replace the chlorine by hydroxyl.
Similar sulphur compounds, however, are stable. Methyl
sulphide, produced by the action of methyl iodide on potas-
CH3I 1C KI CH3,
sium sulphide, + J>S — + J>S, unites with
CHJ K/ KI CH/
ETHERS 91
^
methyl iodide, forming /S\ ? a compound con-
CH/ \I
taining tetrad sulphur ; with silver hydroxide it yields
the corresponding tri-methyl-sulphonium hydroxide,
CH3 CH3
"
, a compound exhibiting basic properties.
OH
From iodine, too, iodonium compounds have been
prepared, in which the iodine functions as a triad ; and a
hydroxide with basic properties is known.
CHAPTER V
Neutral Oxides — Peroxides— Action of Nitric
Acid on Metals; on Oxidisable Substances
— Complexity of Oxides — Spinels and Simi-
lar Compounds.
THE properties of all chemical compounds show gradation ;
and there is a slow transition from basic oxides and hydr-
oxides, like those which we have been considering in the
last chapter, to acid oxides and hydroxides. The transition
takes place along two paths ; first, there are some oxides
which are neither basic nor acid ; and second, a number of
oxides exist which are either basic or acid, according to
circumstances. We shall consider first the neutral oxides.
Peroxides. — In the potassium and calcium groups of
elements, peroxides are known. When sodium is burned
in air a light yellow powder is formed, sodium dioxide,
of the formula Na2O2 ; potassium yields a tetroxide,
K2O4. Both of these substances react with water, giving
off oxygen ; but if they are very slowly added to the water,
so that the temperature does not rise much, a solution is
obtained. The corresponding barium compound is formed
when barium monoxide is heated under pressure in air (see
p. 13). On addition to water it forms a hydrate, probably
Ba=O(OH)2.7H2O. On treatment with acid, hydrogen
dioxide, H2O2, is formed ; and if sulphuric acid be added
in theoretical amount to the barium dioxide, nearly
insoluble barium sulphate is formed, along with a fairly
pure solution of hydrogen dioxide : BaO=(OH)2.Aq
NEUTRAL OXIDES 93
+ H2SO4. Aq = BaSO4 + O=OH2. Aq. It can be purified,
and indeed obtained anhydrous by distillation under very low
pressure. It then forms a somewhat viscous, colourless
liquid, with a sharp taste.
There is some doubt as to the constitution of hydrogen
dioxide, and consequently of the dioxides from which it is
derived. It is unlikely that barium ever acts as a tetrad,
and much more probable that this character is to be attri-
buted to oxygen ; hence the formula of its dioxide is more
likely to be Ba=O=O, than O=Ba=-O ; and consequently
hydrogen dioxide has more probably the formula O=OH2,
than HO=OH. Indeed, hydrogen dioxide is possibly a
weak acid, since the hydrated dioxides of calcium and
barium are precipitated on addition of concentrated solu-
tions of hydrogen dioxide to the hydroxides suspended in
water. These substances have all bleaching power, for they
readily part with their second atom of oxygen, and it is
capable of oxidising coloured insoluble substances to colour-
less soluble ones.
Neutral Oxides, Class I. — The next neutral oxides
met with are carbon monoxide, CO, nitrous oxide,
N2O, and nitric oxide, NO. These are all gases, but
condense at low temperatures to colourless liquids, and at
still lower, freeze to white solids.
Carbon monoxide is prepared by burning carbon in a
supply of oxygen insufficient to convert it into the dioxide ;
or by passing the dioxide over a layer of carbon, heated to
redness. It appears that the monoxide is always the first
product ; for if moisture be excluded during the combustion
of carbon in oxygen, the amount of dioxide relatively to the
monoxide is very small ; and it is known that if water-vapour
be absent, carbon monoxide cannot be induced to explode with
oxygen. If even the minutest amount of moisture be present,
on passing a spark the union takes place with explosion. This
phenomenon is not easily accounted for ; it is readily repre-
sented by the equation 2CO +H2O + O2= 2CO2 + H2O.
Can it be that at the very low pressure of the water-vapour
94 MODERN CHEMISTRY
+
a trace is ionised into H and OH, and that the OH furnishes
the oxygen for the CO, the hydrogen recombining with
oxygen to re-form the molecule of water ? For it has
been found that no moisture is requisite to promote the
union of oxygen and hydrogen if these gases be heated
together. Phosphorus and sulphur, too, show reluctance in
uniting with oxygen, in absence of moisture. In ordinary
moist air, carbon monoxide burns with a blue flame. It is
nearly insoluble in and has no action on water.
Other methods of preparing carbon monoxide are : by
withdrawing the elements of water from formic acid by
adding it drop by drop to warm concentrated sulphuric
O
acid ; HC - OH + H2S04 = CO + H2SO4.H2O ; by heat-
ing a mixture of oxalic acid with concentrated sulphuric
CO. OH
acid ; | + H9S04 = CO + C02 + H9SO4.H2O ;
CO.OH
the carbon dioxide is separated from the monoxide by
bubbling the mixture of gases through a solution of caustic
potash, which absorbs the dioxide, allowing the monoxide
to pass ; and lastly, by heating a mixture of potassium ferro-
cyanide and fairly concentrated sulphuric acid ; K4Fe(CN)6
+ 6H.2S04 + 6H20 = 2K?S04 + FeSO4 + 3 (NHJ,SO4
+ 6CO. In the last reaction, it may be taken that hydro-
cyanic acid, HCN, is first liberated, and that it reacts
with water, forming ammonia and carbon monoxide : HCN
+ H2O = NH3 + CO ; the ammonia subsequently combines
with the sulphuric acid.
If carbon monoxide is passed over metallic nickel or
iron in a fine state of subdivision produced by reducing
their oxides, volatile compounds are formed of the formulas
Ni(CO)4, and Fe(CO)5; on exposing the latter to light
gold-coloured crystals are formed, of the formula Fe2(CO)^.
The nickel carbonyl boils at 43°, and the iron penta-
carbonyl at 103°; di-ferro-hepta-carbonyl decomposes
NITROUS OXIDE 95
when even moderately heated. At 180° these compounds
are decomposed into metal and carbon monoxide, the metal
being deposited as a mirror on the hot surface.
Nitrous oxide, N2O, is most readily prepared by
heating ammonium nitrate, NH4NO3 ; the equation is :
NH4NO3-N2O + 2H2O. It is somewhat soluble in
water, and is best colfected by downward displacement.
The aqueous solution has a sweetish taste ; and the gas, if
breathed, produces insensibility ; it is therefore frequently
employed by dentists as an anaesthetic. If a mixture with
air is respired, it produces with some persons a state of
excitement, which has procured for it the name " laughing-
gas." It is an endothermic compound, and if submitted to
sudden shock it explodes with violence. It may be sup-
posed that the fulminate used to explode it decomposes
some molecules in the neighbourhood ; these, on decompos-
ing, evolve heat, and decompose their neighbours, and the
explosion rapidly travels throughout the gas ; the products
are nitrogen and oxygen. A candle will burn in nitrous
oxide, for the temperature of the flame is sufficiently high
to decompose the gas, and the combustion proceeds as in
dilute oxygen. Although nitrous oxide is not acted on by
water or bases it has claims to be regarded as the anhydride
of hyponitrous acid, from a solution of which it is liberated
N-OH N,
by heat: || - li \O+H9O. As neither ammo-
N-OH N/
nium nitrate nor hyponitrous acid can be reproduced by
bringing together nitrous oxide and water, its production by
heating one of these compounds is termed an " irreversible
reaction."
Action of Nitric Acid on Metals. — The product
of the action of nitric acid on metals varies according to the
metal acted on, the concentration of the acid, and the
temperature. The acid in aqueous solution is more or less
-f
ionised, the ions being H and NO3. If a metal of which
96 MODERN CHEMISTRY
the ions are highly electropositive is presented to these ions
of nitric acid the hydrogen ions impart their charge to the
non-ionised metal, which metal enters into solution as ions,
while hydrogen is evolved. This is the case when nitric
acid acts on magnesium, and theoretically also on aluminium,
manganese, zinc, cadmium, iron, cobalt, and nickel, for all
these metals in the ionic state have higher electro-affinity
than hydrogen, and that in the order given. It may be
termed the normal action of acids on metals, and repre-
+ +
sented thus : M + 2H = M + H2. But along with this
action others take place in which the nitric ion is " re-
duced " or deprived of oxygen. Some examples of this
will now be given.
When silver is attacked by nitric acid, nitric peroxide,
NO9, is produced, and partly evolved as gas. The react-
H NO3
ing substances are Ag, and + and ; one of the
H N03
NO3 groups loses oxygen, being converted into electrically
neutral NO2 and an ion of oxygen, O, which combines
with the two hydrogen ions, forming water, non-ionised,
H2O. But this leaves a negatively charged nitrate group
without a corresponding positively charged partner ; more-
over, the charge of the decomposed nitrate group is still
available. An atom of silver, therefore, goes into solution
as a positively charged ion, and restores electric equilibrium
in the solution. With less concentrated acid the nitrate ion
parts with two atoms of oxygen, requiring three negative
electrons, in addition to the one originally attached to the
group NO3 ; to effect this three positive electrons must
attach themselves to three atoms of silver, which then go
+
into solution as ions, hence the charge is : 3 Ag + 4-H +
4NO3 = NO + 2H2O + 3 Ag + 3NO3, the balance of elec-
OXIDES OF NITROGEN 97
trie charge not having been disturbed, although one nega-
tive and one positive electron have disappeared. With
metals yielding kations of higher potential, the reduction of
the nitrate ion goes still farther ; nitrous oxide, N2O,
nitrogen, and even ammonia may be produced, in relative
amounts depending on the metal, on the concentration, and
on the temperature. It may be taken that the lower the tem-
perature, the less the concentration, and the higher the metal
stands in the electro-negative series, the greater the reduc-
+
tion. The equations are : qM" + loH + ioNO3 = N2O
i2NO = N
2
6H,0 + 5 M+ i oN03 ; 4M" + i oH + i oNO3 = NH4 +
"
. All these changes may proceed
simultaneously ; but copper and moderately strong nitric
acid yields fairly pure nitric oxide ; if more concentrated
acid be employed, a mixture of varying proportions of nitric
oxide and peroxide are evolved ; while by using zinc or
magnesium and very dilute acid, nitrous oxide, nitro-
gen, hydrogen, and ammonium nitrate are the main
products.
Oxidation by means of Nitric Acid. — Action of
the same nature occurs when an element capable of chang-
ing its valency, i.e. the number of electrons associated with
its ionised atom, is treated in the ionic condition with nitric
+ +
acid. For example, the ferrous ion, Fe, on treatment
+ + +
with nitric acid at 100° becomes ferric, Fe, while nitric
+ + +
oxide is evolved : 3 Fe + 6R + 4H + 4NO3 = NO + 2H2O
+ + +
+ 3Fe+ 3NO3 + 6R ; R being any monovalent anion.
Such operations are usually spoken of as " oxidations in
the wet way/'
Nitric oxide is a colourless gas, very sparingly soluble
in water ; on bringing it into contact with oxygen, unless
VOL. II. G
98 MODERN CHEMISTRY
moisture is absolutely excluded, union takes place to form
nitric peroxide, NO2, along with a trace of N2O3, nitrous
anhydride. On sufficiently cooling nitric oxide it con-
denses to a colourless liquid, and at a still lower temperature
it forms a white solid.
Nitrous anhydride, strictly speaking, belongs to the class
of acid-forming oxides ; its formula is N2O3, When nitric
oxide and nitric peroxide are brought together, only a minute
quantity of N2O,} is formed ; that is, because on converting
it into the gaseous state it decomposes almost completely
into these products. On cooling such a mixture, however,
a blue liquid condenses, which has the formula N9Og. It
will be afterwards alluded to.
Nitric peroxide, as usually seen mixed with air at
ordinary temperatures, is an orange-coloured gas. When
pure it condenses to an orange-red liquid, boiling at 22° ;
it freezes at — 10° to a colourless solid. The liquid has a
molecular weight corresponding to the formula N2O4, and
the gas, at temperatures not much exceeding the boiling-
point, consists mainly of the same substance. But as the
temperature rises the colour grows darker, until, at 140°, it
forms a blackish-red gas, consisting wholly of NO2. With
progressive increase of temperature NO2 dissociates in its
turn into NO and O2, and at 600° the change is complete.
As temperature falls the action is reversed.
Neutral Oxides, Class II. — The next class of
oxides comprises those which may be termed neutral,
because they can act either as bases or as acids, according
as they are treated with an acid or with a base. Their
hydroxides may be comprised in the same class. A case
of this kind has already been explained on p. 70 ; it is
there shown that aluminium hydroxide, when treated with
acids, yields salts of aluminium, while with bases aluminates
are formed.
Complexity. — It appears probable that such oxides
have molecular formulae more complex than those usually
ascribed to them; for instance, aluminium oxide is certainly
NEUTRAL OXIDES 99
more complex than is implied by the usual formula A19O3 ;
it may be A14O6 or A16O9, but there is no means at present
of determining the degree of complexity of the molecule.
The argument in favour of this view is the very high
melting-points and boiling-points of such oxides. It is a
well-known fact that as the molecular weight of compounds
increases the boiling-point rises. Examples to illustrate
this are best drawn from carbon compounds, where " poly-
merism" is not infrequent ; that is, where compounds exist
having the same percentage composition but molecular
formulas, of which the higher ones are multiples of the
lower one. We are acquainted with a series of com-
pounds of carbon and hydrogen, of which the first member
is ethylene, C2H4 ; bodies of the formulae C4Hg, C6H12,
C8H16, C10H20, &c., are also known ; and the boiling-point
increases with the molecular weight. Now, the chlorides
of the elements are, as a rule, easily volatile, and have low
melting-points ; and where it happens that both chloride and
oxide have a simple molecular formula, as, for example,
carbon tetrachloride, CC14, and carbon dioxide, CO9, the
chloride has always a higher boiling-point than the oxide.
It would appear to follow, therefore, that if the oxides of
the metals had as simple molecular formulae as the chlorides
they would show more volatility than the latter. As this
is not the case, the presumption is that the oxides possess
more complex formulae than we are in the habit of ascribing
to them. This probability will be dealt with as occasion
arises.
Among the oxides and hydroxides which exhibit the
power of acting both as acid and basic compounds are
cupric hydroxide, Cu(OH)2, which dissolves in a con-
centrated solution of potassium hydroxide with a dark blue
colour ; zinc and cadmium hydroxides, which dissolve
in excess of alkali ; sodium zincate has been separated by
addition of alcohol, and is precipitated in white needles of
the formula Na2ZnO2.8H2O; and aluminium hydroxide,
which dissolves in alkali, forming an aluminate, MA1O2 ;
ioo MODERN CHEMISTRY
stannous and plumbous hydroxides, Sn(OH)2 and
Pb(OH)2, dissolve in alkalies, forming Compounds no
doubt analogous to zincates. Chromous, ferrous, manganous,
cobaltous, and nickelous hydroxides are not thus soluble.
Chromic hydroxide, however, is soluble in soda, probably
forming a compound like sodium aluminate ; unlike the
latter, chromium hydroxide is thrown down on boiling the
solution.
But such compounds, when they do not contain sodium
or potassium, are often insoluble in water, and then they
cannot be prepared by the action of the one hydroxide on
the other. The oxides combine when heated together in
the dry condition, and sometimes when the compound
formed is decomposed by water (hydrolysed) it is con-
venient to prepare it either from the oxide or from the
carbonate.
Spinels. — A considerable number of compounds,
analogous to the aluminates, is produced in this way, and
many of them are found in nature as minerals. To this
class belong the " spinels," so called because one of their
number, the native aluminate of magnesium, had received
this name. Viewed as a combination of oxides, such com-
pounds possess the general formula M2OrMO, and they
can be prepared by heating the sesquioxide (a name given
to oxides when the proportion between the metal and the
oxygen is as one to one and a half, or, more correctly, as
two to three) with the monoxide. The spinels all crystal-
lise in regular octahedra ; they are therefore said to
be isomorphous with each other. Viewed as aluminates,
they may be written M"(MO9)9 ; compare NaAlO.,.
Among them are true spinel, Mg(AlO2)2; franklinite,
Zn(FeO2)2; chrysoberyl, Be(AlO2)2; and chromite,
or "chrome-iron ore/' Fe(CrO2)2. But it is not neces-
sary that the metals of a spinel should be different ones ;
if a metal is capable of existing in two forms, e.g. as dyad
and triad, it may form a similar compound. Such are
magnetite^ or "magnetic iron ore/7 Fe"(Fe'"O2)2, and
SPINELS • • 101
hausmanite, Mn"(Mn'"C,)<,, th^ lirst atom- of .Iron or
manganese being dyad, like magnesium, and the second
triad, like aluminium.
Reasoning by analogy, it would appear not unlikely that
native oxides, such as alumina (corundum, ruby, sapphire),
or iron sesquioxide (haematite), may be in reality an
aluminate of alumina, A1(A1O0)3, or ferric ferrite,
Fe(FeO,)8.
A common test for zinc and aluminium is to heat
together before the blow-pipe the salt suspected to contain
the metal with cobalt nitrate ; it is probable that the green
colour produced by zinc is due to the formation of a cobalt
zincate, Co(ZnO0), and the blue colour shown by alumina
to a similar body, Co(AlO2)2.
When lead is heated to redness in air the first product
of its oxidation is litharge, PbO ; on continuing the ap-
plication of heat, at a carefully regulated temperature, the
yellow litharge becomes red, and the product of the action
is minium or " red lead," PbgO4. Now, on treating red
lead with dilute nitric acid, lead nitrate dissolves, while
lead dioxide, hydrated, remains as an insoluble residue.
Red lead, therefore, may be regarded as a compound
between two molecules of monoxide and one of dioxide,
2 PbO + PbO2 ; the former reacts with nitric acid forming
the nitrate, while the latter remains. Now, if the dioxide
be heated with caustic potash it dissolves, forming potassium
plumbate, K.JPbO3 ; and red lead may be regarded as a
/Pb"\ iv
basic plumbous plumbate, O<^ VPbOg) ; "basic,"
\pb"/
because the first written atoms of lead are partly oxide,
partly salt ; they are dyad, while the second atom of lead
is tetrad.
It is possible to regard nitric peroxide in this light
as a nitrate of nitrosyl, O=N— NO3 ; but its easy decom-
position into NO2 when heated militates against the
view. Compounds of antimony and bismuth, having the
102 MODERN CHEMISTRY
formula* Sfe-2O4 '.arid Ei,jO44 inay be similarly regarded
as O=Sb(SbO3) and G>=Bi(BiO3) ; of this, however,
there is no proof.
Manganese and chromium also form " dioxides," to
which the simple formulas MnO2 and CrO9 are usually
o ' ,o
attributed ; they, too, may be written x/Ci\ /Cr
O^ XX
o, TI xx
and VMn/ />Mn". They would then be termed
O^ NX
chromous chromate and manganous manganate. Such
ideas must be regarded as speculative, but there can be little
doubt that the formulae are more complex than they are
usually written. The former is a snuff-coloured powder,
produced by the action of nitric oxide on a chromate ; the
latter, formed by oxidising and precipitating a manganous
salt simultaneously, is best prepared in a hydrated state by
the action of a hypobromite on a manganous salt: MnCl9.Aq
4-NaOBr.Aq + 2NaOH.Aq - O=Mn(OH)2 + NaBr.Aq
+ 2NaCl.Aq. It is a common black mineral in the anhy-
drous state, and is known as pyrolusite. It will be re-
membered that the ordinary method of preparing chlorine is
to heat this mineral with dilute hydrochloric acid, and also
that on heating alone it furnishes oxygen, being itself con-
verted into Mn3O4, a brown powder, which may be formu-
lated as a spinel, viz. (O=Mn— O)2=Mn.
In concluding this chapter on neutral oxides, it may be
mentioned that there are a few which, acting generally as
feeble bases, yet display feebly acid properties if in the
presence of a strong base like soda or potash. Such are
the oxides of gold, the metals of the platinum group,
and of titanium, zirconium, and thorium. The chlorides
of these elements are soluble in water, as also the sulphates
and nitrates of the last three. Sulphates of gold and plati-
num, however, are hydrolysed by water, giving oxides and
sulphuric acid, thus : Pt(SO4)2 + zHOH = PtO2 + 2H2SO4.
NEUTRAL OXIDES 103
Salts of these elements, on treatment with soda, yield no pre-
cipitate, for they are dissolved by the alkali ; the compounds
formed are indefinite, but it may be supposed that they
contain aurate, MAuO2.Aq, or platinate, titanate, zirconate,
or thorate, MPtO3.Aq, &c. Iron and calcium titanates
occur native ; FeTiO3 is termed ilmenite, and CaTiO3
perowskite. The first is isomorphous with and crystal-
lises along with native ferric oxide ; the ore is known as
" titanic iron ore." It is the commonest compound of
titanium.
CHAPTER VI
Anhydrides — Acids and Salts — Basic and Acid
Chlorides— The Borates—The Carbonates and
Thiocarbonates — Other Acids containing Car-
bon; their Salts with Alcohol Radicals— The
Silicic Acids and the Silicates.
Basic Salts. — Many compounds are known which are
at the same time chloride and oxide, or chloride and hydr-
oxide of elements. Where the element with which the
oxygen and chlorine is combined is one which forms a basic
oxide, the compounds in question are termed basic chlorides.
Similarly, there are basic bromides and iodides. For ex-
ample, zinc oxide heated with zinc chloride forms oxychlo-
rides, of which the simplest example is Cl~~Zn~~O~Zir~Cl;
aluminium chloride, evaporated with water, has its chlo-
rine gradually replaced by hydroxyl, forming successively
C12=A1(OH), C1-A1=(OH)2, and finally, A1(OH)8,
though at a temperature sufficient to complete the reaction,
the aluminium would probably form the condensed hydr-
oxide O=A1OH instead of the trihydroxide. We shall
see later that other groups, playing a part analogous to
that of the chlorine in a basic salt, may also exist in basic
salts.
Acid Chlorides. — Another class of double oxides and
chlorides exists, most of which are easily volatile, and
which therefore are of known molecular weight. These
are the so-called "acid chlorides" — oxychlorides of those
elements which form acids. These are related to acids, in
as much as by replacement of their chlorine by hydroxyl,
BORATES 105
acids are formed. It will therefore be convenient to con-
sider them along with the acids to which they are related.
A general idea has already been given of the nature of
acids in describing the hydroxides of zinc and of aluminium.
As a rule, acids are condensed hydroxides ; that is,
hydroxides which, having lost the elements of water, are
partly oxides, partly hydroxides. They also possess the
property of ionising into one or more hydrogen ions and an
electro-negatively charged radical. In following the order
of the periodic table, after such feebly acidic hydroxides as
those of zinc and aluminium, hydroxide of boron claims
attention.
Borates. — In certain lakes in California the water,
when evaporated, deposits crystals of the formula
Na2B4OrioH2O ; this substance is named borax. It is a
white, crystalline salt, easily soluble in hot water, but sparingly
soluble in cold. When mixed with sulphuric acid nacreous
scales separate of the formula B(OH)3, or, as it is usual in
writing the formulas of acids to place the hydrogen atoms
first, H3BO3. Boracic acid hardly deserves the name of
acid ; in aqueous solution it exists almost entirely in the
non-ionised state. No ions are volatile ; but this compound
issues in Tuscany and in the Lipari Islands along with
steam from cavities in the ground, termed suffioni ; it is
easily recognised, for it imparts a green colour to a flame
held in the steam. When heated to 100° boracic acid loses
water and is changed into metaboracic acid, O=B-OH,
a vitreous substance ; and at a red-heat boron oxide,
Bi;O3, is left as a transparent, colourless glass. Its con-
stitution is O=B— O— B=O.
The borates of the alkalies are prepared by mixing
boracic acid with the hydroxide of the alkali metal ;
although there are very few hydrogen ions in an aqueous
solution of boracic acid, however dilute, yet some of those
present combine with the hydroxyl ions of the alkali, forming
water, thus: H3BO3.Aq + 3NaOH.Aq = Na3BO3.Aq +
io6 MODERN CHEMISTRY
3H2O. But there are so few ions present, that those of
the water, which, it will be remembered, is ionised, although
to an extremely minute extent, are yet sufficiently numerous
to bear some proportion to those of the boracic acid ; hence
the reaction given above is perceptibly reversed, and on
dissolving borax in water it is " hydrolysed," that is, split
"by the hydrogen and hydroxyl ions of the water into non-
ionised boracic acid and caustic soda, the latter, of course,
largely ionised as usual. It is therefore possible to estimate
the sodium of borax by addition of a solution of a strong
acid, such as hydrochloric or sulphuric acid of known con-
centration, just as if no boracic acid were present, provided
methyl-orange be used as an indicator. (See p. 75.)
Thus the addition of 36.5 grams (H=i; 01 = 35.5)
of hydrochloric acid, dissolved in a litre of water (such a
solution is termed a "normal solution"), to 191 grams of
a solution of crystallised borax in a litre of water (1/2
[Na2 = 46 + B4 = 44 + O7 = 112 + ioH2O = 180]) ; (in
all 1/2 of 382) gives a solution which is neutral to methyl-
orange.
Fused borax has the property of dissolving oxides of the
metals, forming complex borates ; certain of these are
coloured, and their formation is often made use of for
detecting the presence of such metals as copper (blue),
silver (yellow), chromium (green), ferric iron (yellow),
ferrous iron (bottle-green), manganese (amethyst, when
heated in a flame containing excess of oxygen), cobalt
(blue), and nickel (reddish). Borax is also used for
soldering easily oxidisable metals, such as iron, copper, or
brass ; the film of oxide which prevents the metal touching
and alloying with the solder is thus removed. Both borax
and boracic acid have considerable antiseptic properties, and
are used for preserving eggs, milk, and other animal and
vegetable substances.
Carbonates and Thiocarbonates. — The car-
bonates and the thio carbonates are derivatives of carbon
dioxide (or ' rather of carbon oxy-hydroxide, commonly
CARBONATES 107
called carbonic acid), and of carbon disulphide. Carbon is a
tetrad, and. the analogue of carbon tetrachloride would be the
tetrahydroxide, C(OH)4; but this body is unstable, and
its first anhydride, O=C(OH)2, is known only in aqueous
solution. However, carbonyl chloride, O=CC12, exists ;
it is produced by the direct union of carbonic oxide with
chlorine, when a mixture of both gases is exposed to
sunlight ; it was formerly known as " phosgene gas,"
meaning "made by light"; but it is more conveniently
prepared by passing a mixture of the two gases over
animal charcoal heated to redness. It condenses to a
liquid, boiling at 8.4°. It is immediately decomposed by
water, thus: O=CC12 + 2HOH = O=C(OH)2+ 2HC1;
if sufficient water is present, the carbonic acid can remain
in solution. The existence of the oxychloride establishes
the formula of carbonic acid.
Carbonic acid is a very easily decomposable substance ;
if liberated, unless a great deal of water be present, it splits
into its anhydride, CO2, and water: O=C(OH)2 = CO0
+ H2O. The anhydride is a colourless gas, which con-
denses to a solid at about —80° ; it can be liquefied only
under pressure. Carbon dioxide, or carbonic anhydride,
is produced by heating a carbonate ; as already remarked,
all carbonates, except those of the alkaline metals, are
decomposed by heat, forming oxides, and evolving carbon
dioxide. It is also produced when carbon or carbon
monoxide is burned with excess of oxygen. Lastly, it is
produced in large quantities during the process of fermenta-
tion. Glucose, or grape sugar, either produced by the
hydrolysis of starch or extracted from fruits like grapes,
when mixed in dilute aqueous solution with yeast, a vegetable
organism, decomposes into ethyl alcohol and carbon dioxide,
thus: C6H12O6 = 2C2H5OH + 2CO2. The carbon di-
oxide being heavier than air, collects in the fermenting
tuns ; it is now often collected and compressed until it
liquefies ; and the liquid on expansion solidifies to a snow-
like solid, used for producing low temperatures.
io8 MODERN CHEMISTRY
A solution of carbonic anhydride in water contains
carbonic acid, O=C(OH)9, which is a very weak acid
owing to the small extent of its ionisation. It is probable,
too, that liquid carbon dioxide exists in the solution, mixed,
but not combined with the water. Carbonic acid reacts
with sodium, potassium, calcium, or barium hydroxide,
f - - + •
forming carbonate of the metal: H2CO3.Aq + zNaOH.Aq
= Nt2C03.Aq + 2H20 ; H2C03.Aq + Ca(OH)2.Aq -
CaCO3 + 2H9O. In such actions it is only the ionised
portion of the acid which reacts, and the hydrogen ions
form water ; when these are removed another portion
becomes ionised in order to restore equilibrium ; it reacts in
its turn until all has become transformed. On evaporation
of the solution the alkaline carbonate is left as a white crystal-
line salt; hydrated sodium carbonate, Na9CO3. ioH2O,
is ordinary washing-soda. All other carbonates are insol-
uble in water, and are consequently thrown down as
precipitates on adding a solution of sodium carbonate to
any ionised solution of other metals. They form flocculent
precipitates, generally possessing the colour of the ion
of the metal ; thus copper carbonate is blue, ferrous
green, cobalt pink, and so on. But with the exception of
the carbonates of the metals of the sodium and calcium
groups all other precipitated carbonates are " basic," that
is, they are partly hydroxides, partly carbonates. Copper
carbonate, for example, may be assigned the formula
/O-Cu-OH
O=C<f ; it will be noticed that each atom
X0-Cu-OH
of copper is combined with the oxygen of the carbonic
residue on the one hand, and with hydroxyl on the other.
The paint known as " white lead " consists of a basic
carbonate of lead, more complex than the example given
above, of the formula
HO-Pb-0-(CO)-0-Pb-0-(CO)-0-Pb-OH.
Native Carbonates. — Many carbonates exist in the
ACID CARBONATES 109
native state ; some are widely distributed minerals. Among
these are Iceland or calc-spar, arragonite, limestone,
chalk, and marble, all of them calcium carbonate;
strontianite, SrCO3 ; witherite, BaCO3 ; spathic iron
ore, FeCOs, also named clay-band when contaminated
with clay, and black-band when mixed with shale.
Magnesite is MgCO3 ; dolomite, a mixture of magne-
sium and calcium carbonates ; calamine, ZnCO3 ; and
cerussite, PbCO3. Malachite and azurite are basic car-
/O-Cu-OH
bonates of copper, O=C\ , and
X0-Cu-OH
O O
HO-Cu-0-C-O-Cu-O-C-O-Cu-OH.
We see here again that with weak bases, such as the
hydroxides of most metals, the carbonates tend to become
basic, that is, to be hydrolysed. This is why the preci-
pitates obtained on adding a soluble carbonate to a salt of
such metals are basic, and not normal carbonates.
"Acid" Carbonates. — The name "acid carbon-
ate " is given to a double carbonate of hydrogen and a
metal. Such bodies are prepared by the method which
always is adopted for the preparation of double salts — by
/ONa
mixture. Hydrogen sodium carbonate, O=C\ ;
XOH
the corresponding potassium compound ; hydrogen cal-
O O
I! II
cium carbonate HO-C- O-Ca-O-C-OH, a ferrous
carbonate of similar formula, and many others are all
formed when carbonic acid and the respective normal car-
bonate are mixed, the mixture being kept cold. On raising
the temperature of all of these, carbon dioxide escapes, and
the neutral carbonate is again formed. " Acid " carbonate
of sodium is the common " baking-soda ; " hydrogen calcium
no MODERN CHEMISTRY
carbonate is a constituent of many natural waters, and is the
cause of what is termed " temporary hardness " ; for on
boiling the water the neutral carbonate is precipitated, and
the water ceases to be "hard." The same result may be
effected, paradoxical as it may appear, by the addition of
lime-water ; for then sufficient calcium hydroxide is present
to form normal calcium carbonate with the hydrogen carbon-
ate, thus : Ca(HCO3)2.Aq + Ca(OH)2.Aq = 2CaCO3 +
2H2O.Aq. Hydrogen ferrous carbonate is a constituent
of chalybeate springs ; on exposure to the atmosphere the
iron is oxidised to ferric hydroxide, and the carbonic acid,
being too weak an acid to form a carbonate with such a
weak base as that, escapes: 2Fe(HCO3)2.Aq+ 5H2O +
O = 2Fe(OH)3 + 4H2CO3.Aq. The ferric hydroxide is
deposited as a brown scum on the banks of the streams
flowing from such wells.
Carbonates of Radicals. — Although normal hydr-
oxide of carbon is unknown, yet if the hydrogen be replaced
by ethyl, — C2H5, the compound is stable. The compound,
which is produced by the action of carbon tetrachloride
on sodium ethoxide, CC14 + 4Na-O-Ck>H5 = 4NaCl +
C(O-C2H5)4, is the analogue of C(OH)4. It is a
volatile liquid, and is named ethyl orthocarbonate. And
a corresponding carbonate of ethyl, O=C(OC2H5)2, the
analogue of carbonic acid, O=C(OH)2, is formed by treating
carbonyl chloride 'with alcohol: O=CCU + 2HO-C2H5=
O=C(OC2H5)2 + 2HC1. These compounds are volatile,
and can be weighed in the state of vapour, hence their
molecular weights are known, and this is an additional
proof of the correctness of the formulae ascribed to carbonic
acid and the carbonates.
Thiocarbonates. — The sulphocarbonates, or thiocar-
bonates (from the Greek theion, sulphur) form a class
of salts analogous to the carbonates, both in their formulas
and in the method of their preparation. Carbon disulphide,
a volatile liquid, boiling at 46°, possessing a disagreeable
smell, is produced when sulphur vapour is led over charcoal
THIOCARBONATES in
heated to redness in a fireclay tube ; in fact, the carbon is
burned in sulphur gas. When shaken with a concentrated
aqueous solution of the sulphide of sodium or potassium, it
dissolves, forming the compound Na2CS3, or K9CS3.
These thiocarbonates, like the carbonates, are white, crystal-
line salts ; on adding acid, thiocarbonic acid separates as
an oil ; it slowly decomposes, especially if warmed, into
hydrogen sulphide and carbon disulphide. Many of its salts
are insoluble, and may be prepared by precipitation.
The formula of carbon dioxide is CO2, that of carbon
disulphide CS2 ; and it is evident that an intermediate
substance should exist of the formula COS. This sub-
stance is carbon oxysulphide. It is a gas, prepared by
heating thiocyanic acid, HSCN, the ammonium salt of
which is produced when ammonia is passed through a
mixture of carbon disulphide and alcohol : CS2 + 2NH3.Alc
= H2S + (NHJSCN.Alc. On evaporation of the alcohol
the ammonium thiocyanate crystallises out. This salt, dis-
tilled with sulphuric acid, yields in passing the acid HSCN,
which, on account of the high temperature, reacts with
water, forming ammonia (which yields ammonium sulphate
with the sulphuric acid) and carbon oxysulphide, COS :
HSCN + H2O - NH3 + COS.
Like nitrous oxide, carbon disulphide is an endothermic
compound, and can consequently be decomposed by shock ;
when a fulminate is exploded in it, it is resolved into carbon
and sulphur. On the other hand, carbon dioxide and oxy-
sulphide are exothermic compounds, heat being evolved
during their formation.
Acids containing Carbon. — An enormous number
of acids containing carbon is known, in which the acidic
carbon atom is combined with oxygen and hydroxyl, and
also with hydrocarbon residues, such as methyl or ethyl, or with
some more complex group of carbon atoms. The simplest
O
of these is formic acid, H— C— OH. Acetic acid is
ii2 MODERN CHEMISTRY
O
II
methyl-formic acid (CH3)— C— OH ; ethyl-formic acid is
O
ll
named propionic acid ; its formula is CH3— CH2— C— OH.
0=>C-OH
Oxalic acid is to be regarded as di-carboxyl, I ,
O=C-OH
the name carboxyl being a contracted form of" carb(onyl
hydr)oxyl" ; it is commonly written —CO— OH.
Formic acid (from formica, an ant) is contained in ants
and stinging nettles. Sodium formate is produced when
carbon monoxide is left in contact with sodium hydroxide ;
the reaction takes a considerable time: CO+NaOH =
H— CO— ONa. It is also formed by heating oxalic acid,
better in presence of glycerine: (CO— OH)2 = CO0 +
H— CO— OH. It is a colourless, pungently smelling liquid,
boiling at 99°, and a fairly strong acid in aqueous solution ;
it is poisonous. Its salts are crystalline, and possess the
colours of the metallic ions which they contain. When
warmed with concentrated sulphuric acid, or with other
substances capable of withdrawing water, it yields carbon
monoxide. Yet CO is not the true anhydride of formic
acid, seeing that an anhydride can be obtained only from
loss of the elements of water from hydroxyl groups, for
formic acid contains the group H— C= ; the real anhydride
o o
would be H— C— O— C— H ; it is unknown.
Acetic acid is the acid constituent of vinegar, and is a
solid, melting at 17° to a liquid, boiling at 118°. It can
be formed synthetically by bringing into contact carbon
dioxide and sodium methide, a compound of the formula
Na-CH3 ; the equation is : Na-CH3 + CO2 =
H3C— CO— ONa ; the sodium salt, distilled with sulphuric
acid, yields acetic acid. It is produced on a large scale by
the distillation of wood ; the distillate consists mainly of
"ORGANIC ACIDS" 113
acetic acid and methyl alcohol ; it is neutralised with lime,
and distilled, when the alcohol passes over, leaving behind
the calcium acetate ; this is evaporated to dryness, and
heated, so as to char tarry matters, also produced when
wood is distilled ; the calcium salt is finally distilled with
sulphuric acid. Acetic acid is also formed by the
oxidation of aldehyde (p. 88), which is itself an oxi-
dation-product of alcohol. The connection between
these bodies is : CH3-CH2-OH, CH3-CH=O, and
O
ii
CH3— C— OH. Aldehyde may be regarded as the
anhydride of CHg— CH=(OH)2, and acetic acid of
CH3 = C(OH)3. The usual oxidising agent is chromic
acid ; if the product of oxidation is conveyed away as it is
formed by sloping the condenser downwards, aldehyde is
obtained ; if the aldehyde is returned to the oxidising
mixture by sloping the condenser upwards, and cooling with
ice and water, the product is acetic acid. The oxidation is
also effected by an organism called " mother of vinegar " ;
sour wine or beer is allowed to trickle down a cask filled
with shavings of beech-wood, on which the slimy masses
of the organism are growing ; oxygen enters, and the vinegar
flows out at the bottom of the cask.
On distilling acetic acid with phosphorus pentachloride,
hydroxyl is exchanged for chlorine : 4CH3— CO— OH +
PC15 = 4CH3-CO-C1 + H8PO4 + HC1. the compound
obtained is named acetyl chloride ; acetic acid may be
regarded as hydroxide of the group (CHg— C=O)— , and
on treating acetyl chloride with water it is at once formed :
CH3-COC1 + H-OH = CH3-CO-OH + HC1.
And aldehyde may be regarded as a hydride of acetyl,
(CH3— CO)— H. A similar body cannot be made
from formic acid, for it decomposes into carbon monoxide
and hydrogen chloride : H-CO-C1 = CO + HC1.
Oxalic acid is contained as hydrogen-potassium salt in
the plants sorrel and rumex. It can be prepared by the
ii4 MODERN CHEMISTRY
oxidation of sugar with concentrated nitric acid, or by
heating sawdust with a mixture of caustic soda and potash
in shallow trays ; on treating the charred residue with
water, sodium oxalate, a comparatively insoluble salt, remains,
while the excess of alkali dissolves ; the sodium oxalate
is extracted with boiling water, and calcium chloride is
added ; this precipitates the almost insoluble calcium
oxalate ; and on digesting it with the equivalent amount
of sulphuric acid, sparingly soluble calcium sulphate remains,
while oxalic acid dissolves. The filtered solution, when
C(OH)3
evaporated, deposits crystals of ortho-oxalic acid, |
C(OH)3
CO-OH
which, at 100°, dehydrate to I . Oxalic acid is
CO-OH
a di-basic acid, and its salts, like those of formic and acetic
acids, have the colour of the positive ion. It cannot be
dehydrated further, for the anhydride, which should possess
CO
the formula P>O, decomposes into CO and CO2.
CO
Salts of these acids with alcohol radicals, such as
methyl and ethyl, are prepared by saturating a solution of
the acid in the respective alcohol with hydrogen chloride,
and then distilling: (COOH)2 + 2CH3OH = (COOCH3)9
+ 2HOH. The hydrogen chloride serves to withdraw
water, and prevent it acting on the product. Such salts5
which are generally colourless liquids or solids, possessing
a pleasant smell, are called " esters.7' As a rule they are
sparingly soluble in water, and are not ionised in solution,
thus differing from salts of the metals. When boiled with
alkalies, the ester being returned by means of an inverted
condenser into the boiling-flask, they change into salts of the
alkalies, and the alcohol: CH0-CO-O-CH2-CH3 +
KOH.Aq ---. CH3-CO-OK.Aq + CH3-CHL-OH.
This change is also effected by heating with water in a
SILICIC ACIDS 115
sealed tube ; it is accelerated by the presence of hydrogen
ions, and therefore by the presence of strong acids, such as
hydrochloric acid. Decomposition of this kind by alkalies
is called " saponification " ; if effected by water the term
" hydrolysis " is applied to it.
Silicic Acids and Silicates. — While the character-
istic of carbon is to form compounds in which many atoms
of carbon are linked together (hydrocarbons, for example,
H H H H H
having formulse like H-C-C-C- ... -C-C-H),
H H H H H
atoms of silicon are characterised by linking by means of
atoms of oxygen. This peculiarity leads to the existence
of a large number of silicates, and probably, too, of a
large number of silicic acids. The existence of some of
these is rendered certain by a study of the oxychlorides.
Silicon tetrachloride, SiCl4, when passed over fragments
of felspar (a silicate of aluminium and calcium) heated to
whiteness in a porcelain tube, exchanges chlorine for oxygen,
and yields a liquid boiling at about 137°, of the formula
/SiCl,
O<Q . This liquid, passed along with oxygen through
\SiCl,
a hot glass tube, gave two other liquids, which could be
separated by fractionation ; the one, boiling at 153°, had
the formula Si4(XCl10, and the other, boiling at 200°,
Si4O4Clg. The vapour-densities of these liquids were
determined, and led to the formulae given above. The
signification of this will appear presently.
Si(OH)4. When silica, in the form of flint, or fine
sand, or powdered rock-crystal is either fused with caustic
soda or potash, or heated under pressure with a solution
of one or other of the alkalies, an orthosilicate is pro-
duced, possessing the formula Si(ONa)4 or Si(OK)4.
These silicates are soluble in water, and as they resemble
glass in appearance, they are usually named " soluble glass."
If hydrochloric acid is added to the solution of one of
ii6 MODERN CHEMISTRY
them, no apparent change occurs ; in reality, orthosilicic
acid is produced, a compound which is hardly ionised at
all, being one of the very weakest of acids.
Osmosis. — To separate the ions of sodium chloride
and of hydrochloric acid advantage is taken of a discovery
made by Graham, that a vegetable or animal membrane like
parchment or parchmentised paper is readily permeated by
crystalline bodies, while it is very slowly permeated by
" colloidal " or gum-like compounds. By placing in a
drum, floating on water, the mixture of orthosilicic acid
and salt, the sodium and chlorine ions pass through, of
course in equivalent proportions, leaving the colloid behind.
Fresh water is substituted from time to time, until all
chlorine ions have been removed from the silicic acid. The
water can be removed by evaporation in vacua, and a clear
but very viscous liquid remains, which is believed to con-
tain Si(OH)4.Aq. On raising the temperature of this
viscous liquid it gelatinises, and is then insoluble in water ;
the resulting compound may have a formula analogous to
carbonic acid, O=Si(OH)2; it is termed metasilicic
acid. On further drying, water is gradually expelled, and
finally a flint-like mass is left, which on ignition yields
a white powder of SiO2, or silica. As already mentioned,
silica is found in nature ; when pure, it crystallises in
hexagonal prisms, and is termed quartz, rock-crystal, or
Irish diamond. It is used for spectacle lenses and optical
instruments.
The major part of the rocks which constitute the
surface of the earth consists of mixtures of silicates.
Occasionally they are found in definite crystals, and on
analysis their formulae can be determined. From their
formulas, the formulas of the silicic acids from which
they may be supposed to be derived can be deduced ;
and tables follow, in which the formulas of these silicic
acids and of some of the minerals constituting their salts
are given.
SILICATES
117
OH
OH
OH
Si
O
O
0\
Be
>Be
Beryl.
Al-Si04-Al
Xenolite.
Orthosflicates. — These are orthosilicates ; the
comma between the Mg and the Fe means that these
metals can replace each other in any proportions. Xenolite,
it will be observed, is the silicate of a triad metal, aluminium ;
and four atoms of aluminium replace twelve molecules of
hydrogen in three molecules of orthosilicic acid. But
double silicates are common, in which three of the atoms of
hydrogen in three molecules of orthosilicic acid may be
replaced by three monad atoms ; or by one dyad and one
monad atom ; or we may have a monad group, such as
-A1=O, or — A1F2, replacing each atom of hydrogen ;
or, lastly, the aluminium may be partly hydroxide, thus
constituting a basic silicate. Examples of such compounds
Al-SiO4-Al
Muscovite, or Potash mica.
Al-SiO4__CaH
Prehnite.
ii8 MODERN CHEMISTRY
Si04-(Al = 0)3 Si04^(AlF2)3
Al— SiO4~Al Al—SiCX-Al
SiO4-Al
Fibrolite. Topaz.
OH
SiO4_Al
Kaolin or China-clay.
In such silicates, the aluminium is often partially replaced
by triad metals, such as triad chromium, iron, or man-
ganese.
Metasilicates. — Metasilicates are derivatives of the
acid H2SiO3 ; the constitutional formula is O=Si(OH)2,
like that of carbonic acid. Examples of metasilicates
are : —
O=Si Ca O = Si (Mg, Fe", Mn", Ca)
\0/ \0/
Wollastonite. Augite or Hornblende.
O °
/ \ il
O = Si Al - O - Si - ONa
Jade.
The potassium salt is Leucite.
Disilicates. — The molecules of orthosilicic acid may
lose one molecule of water, the remaining atom of oxygen
of the two hydroxl groups serving to unite the two molecules
SILICATES
119
together, and a similar loss of water may be repeated twice,
thus : —
orthosilicic acid.
OH
NOH
2nd anhydride.
Si(X
Si00
Silica.
The final loss of water yields anhydrous silica. These
acids are not known as such ; but certain minerals may be
regarded as their salts. It is probable that okenite is a
disilicate, thus CaH4Si2O7 ; and also petalite, a derivative
of its second anhydride, AlLi(Si9O5)2. Similarly, three
molecules of orthosilicic acid, by losing two molecules of
water, may unite to form trisilicic acid ; and it again by
successive loss of one, two, and three molecules of water
may yield a first, a second, and a third anhydride. The
120 MODERN CHEMISTRY
well-known felspars albite and orthoclase are salts of the
third anhydride of trisilicic acid, thus : —
XX O O
/ \ II II
Al— O Si -O-Si-O-Si- ONa
N^
Albite.
7o. o o
/ \ II II
Al -O— Si - O - Si - O - Si - OK
\r
Orthoclase.
The method of ascertaining the formula of a silicate
requires notice. In order to determine the relative number
of molecules of silica, SiO2, and of the oxides of the various
metals in combination with it, each percentage is divided
by the molecular weight of the oxide in question ; the
quotients are then divided by the smallest, and the ratio
between the resulting quotients compared. To take an
instance : —
An analysis of a specimen of muscovite gave the toilow-
ing numbers : —
SiO2 = 45.07 per cent.; A12O8 = 38.41 ; K2O = 12.10;
H9O = 4.42; together = i oo.oo. Dividing by 60.4;
by 102.3 ; by 94.3 ; by 18.0, the quotients 0.746, 0.375,
0.128, 0.245 are obtained. Again dividing the quotients
by 0.128, the smallest of these quotients, the ratios are :
6, 3, i, 2, nearly. Therefore the formula is 6SiO9,
3A12O3, K2O, 2H2O, or, adding the various constituents
together and dividing by 2, Si3O10Al3KH2. The group
Si3O10 is 3 x SiO4 ; the compound is, therefore, an ortho-
silicate ; and three atoms of triad aluminium, one of monad
potassium, and two of monad hydrogen are equivalent to
the twelve atoms of hydrogen of the three molecules of
orthosilicic acid. It sometimes happens, however, that one
SILICATES 121
metal, such as magnesium, may replace more or less of
another, such as calcium and iron. In that case, the
quotients obtained on dividing the percentages by the mole-
cular weights are added before the final ratio is obtained.
The products of Nature's laboratory are seldom, if ever,
pure ; and it rarely happens that a natural mineral gives
results so easily interpreted as the case given above. For
the mineral no doubt separates from a matrix in which
many compounds are present ; and so it happens that one
metal may take the place of another posses-sing the same
valency, and capable of forming compounds of the same
crystalline form. The alkali metals are mutually replace-
able ; also the triads Al, Fe, Mn, Cr, and others. There
are even instances where silicon is partially replaced by
titanium ; hence the interpretation of the results of analyses
presents a problem of no small difficulty. The work of
F. W. Clarke, of the U.S. Geological Survey, has con-
tributed not a little to a solution of this problem.
CHAPTER VII
ANHYDRIDES, ACIDS, AND SALTS
Phosphoric, Vanadic, Arsenic, and Antimonic Acids
— Nitrous, Phosphorous, Arsenious Acids — Phos*
phatic Acid — Hyponitrous and Hypophosphorous
Acids, and their Salts.
THE remaining hydroxides, which all exhibit well-marked
acid functions, may be divided into two classes, those of
elements of odd valency, and those derived from elements
of even valency. A scheme has already been given on
p. 71, which illustrates the derivation of the acids of the
halogens from the imaginary hydroxides corresponding to
Xvn, Xv, X111, and X1, where X stands for halogen, and
the Roman numerals for the valencies in the compounds.
Elements of Odd Valency. — The highest valency
shown by elements of the nitrogen group, apart from a
somewhat questionable pernitric acid, is five. This is
illustrated by the formulx of the pentoxides, N0O5, P9O5,
As2O5, Sb2O5, and V2O5. But these compounds possess
very different stability, and the elements show different
behaviour in uniting with oxygen. Nitrogen and oxygen
do not unite except when electric sparks are passed through
a mixture of the two gases, or when a high-tension current
is passed through air. It is doubtful whether heat alone
is the cause of this union ; it is more probable that energy
must be imparted to the combining gases in an electrical
form. The act of combination, in which the product is
the peroxide NO2, is attended by absorption of heat (7700
PHOSPHORIC ACIDS 123
calories for the union of 14 grams of nitrogen with 32 grams
of oxygen) ; and this energy must be supplied if union is
to take place. On the other hand, phosphorus burns
brilliantly in air, and if excess of oxygen be present, the
so-called pentoxide is produced ; according to the vapour-
density, however, the formula is P4O10. If air be slowly
passed over heated phosphorus, on the other hand, the
lower oxides P4O6 and P0O4 are formed. It is not pos-
sible to dehydrate phosphoric acid, HPO3, completely, so
as to obtain P0O5. When arsenic burns in air, arsenious
oxide, As4Ofi, is the product ; with antimony, Sb4O6 ; but
vanadium pentoxide, V2O5, is formed when the element
or one of the lower oxides is heated in air.
The pentoxides behave differently when treated with
water. While N2O5 and P4O10 unite with water with a
hissing noise to form HNO3 and HPO3, As4O10 slowly
reacts to produce H3AsO4, and probably H3VO4 is the
result of dissolving V0O5 in water ; the corresponding
Sb9O5 is insoluble in water.
Acid Chlorides. — The clue to the constitution of the
acids of these elements is aiforded by the oxychlorides, as
in the case of carbon and silicon. No oxychloride con-
taining pentad nitrogen is known, but phosphoryl chloride,
O=PC13, and antimonyl chloride, O=SbCl3, are pro-
duced by the action of a small quantity of water on the
respective pentachlorides : C12=PC13 + H2O - O=PC13 +
2HC1. The former is a colourless liquid boiling at 107°,
and the latter a white, crystalline solid. Phosphoryl
chloride reacts with hydrogen sulphide, yielding the corre-
sponding phosphoryl sulphide : O=PC13 + H2S - S=PC13
4- H2O, and hydrogen sulphide acts on antimony pen-
tachloride, with formation of S=SbCl3. O=VC13 is
produced by direct union of VO with chlorine. It is a
yellow liquid boiling at 137°. The vapour-densities of
phosphoryl and vanadyl chlorides lead to the ascribed
formulas.
Ortho~, Pyro=, and Meta-Acids. — With water,
i24 MODERN CHEMISTRY
these substances exchange chlorine for hydroxyl, thus :
O=PC13 + 3H-OH - O=P(OH)3 + sHCl. This
establishes the formula of ortho-phosphoric acid. The
name ought, in strictness, to be applied to P(OH)5; but,
as the true ortho-phosphoric acid is unknown, it has been
transferred to what should be termed its first anhydride.
The corresponding nitric acid is unknown. We have thus
the series: O=PCL, O=P(OH)S, O=As(OH)o, and
0=Sb(OH)3.
On heating these bodies the elements of water are lost, and
the " meta-acids " are formed; at a temperature of about
O
200°, O=P(OH)3 yields P-OH, and O=As(OH)3,
II
O
O
II
As — OH ; the former is a glass, the latter a pearly
II
O
substance. On adding water to metaphosphoric acid
it dissolves as such, and, on neutralisation, it yields a
series of metaphosphates ; but metarsenic acid, when
treated with water, is reconverted into orthoarsenic acid ;
a similar change can be produced with metaphosphoric
acid, but only after prolonged boiling.
Dl=Acids. — We are unacquainted with any normal di-
acid of this group, but a number of anhydrides are known.
If Z stand for any element of this group, the series should
run as follows : —
/OH OH
2=
\OH \OH
Di-acid. ist Anhydride.
PHOSPHORIC ACIDS 125
/OH 0=Z— OH
| \
o o
=-
\OH O=Z— OH
2nd Andryhide. 3rd Anhydride.
Neither the di-acid nor the first anhydride are known
in any case, but the second anhydride, which is generally
called the "pyro" acid, because it is formed in certain
cases by heating the " ortho " acid, is known with phos-
phorus, arsenic, antimony, and vanadium. Pyrophos-
plioric acid is formed at 215°; but the change is not
complete, and if a higher temperature be employed the
meta-acid is also produced. Pyroarsenic acid is formed
by heating the ortho-acid to 140°-! 60°. Pyroantimonic
acid, however, is best prepared by the action of boiling
water on antimonyl chloride, O=SbCl3 ; the ortho-acid,
which is probably formed first, loses the elements of water,
forming the pyro-acid, H2Sb2O7. Pyro-phosphoric acid
is a syrupy glass-like substance ; pyro-arsenic acid forms
hard crystals, and pyro-antimonic acid is a sparingly soluble
white powder.
Basicity. — The basicity of these acids is deduced from
the formulas of double salts. Thus, there are three ortho-
phosphates of sodium and hydrogen ; they have the
formulae H2NaPO4, HNa2PO4, and Na3PO4 ; the hydro-
gen calcium salts are: H4Ca(PO4)9, HCaPO4, and
Ca3(PO4)2. It is therefore argued that since the hydro-
gen atoms of ortho-phosphoric acid are replaceable in three
stages by metals, there are therefore three atoms of hydro-
gen. These salts are made by mixture ; 2HHPO4.Aq +
Na3PO4.Aq = 3H2NaPO4.Aq, and so on. The acid is,
therefore, said to be tri-basic. The arsenates are precisely
similar ; but only simple vanadates are known, and no
ortho-antimonates. A pyrophosphate is known of the
formula HK2(NH4)P2O7, which demonstrates the tetra-
126 MODERN CHEMISTRY
basic character of pyrophosphoric acid, and the other
pyro-acids are classified accordingly.
Metaphosphoric Acids. — The formula of the third
anhydride of the di-acid, H2Z.7O0, given on the pre-
ceding page, is a multiple by two of that of the meta-
acid, and it is evident that the fourth anhydride of the
tri-acid, the fifth of the tetra-acid, and so on, will all
be multiples of the simpler formula of the meta-acid.
These acids and some of their salts appear to be known
in the case of the phosphoric acids, and what is usually
termed " meta-phosphoric acid," and given the formula
HPO3, is probably the seventh anhydride of hexa-phosphoric
acid, HrP6O18, for one of its double salts has the formula
Na2Ca5(P6018).2.
Complex salts are known in the case of almost all
these elements. Among such compounds are : H2N4O11,
Ag6P4O13, Ag12P10O31, Na6Vf)O17 ; while niobates and
tantalates are particularly prone to form such complex salts.
Compounds of a complicated kind, too, in which one of
these elements replaces another partially, have been made ;
as an example, K(.(P8V12)O60.2iH9O may be instanced.
They are made by mixture.
The source of the nitrates is the attack of ammonium
salts resulting from the decomposition of urea (the chief
form in which all animals part with the nitrogen they
absorb as a constituent of their food) by a bacterium named
the " nitrate ferment." This organism exists only in the
dark ; it is an inhabitant of the soil, and causes the oxida-
tion of ammonia, from whatever source, into a nitrate. As
potash and lime are the commonest bases in the soil, nitrates
of potassium and calcium are the chief compounds formed.
Vast tracts of country in Chili and Peru contain beds of
sodium nitrate, possibly formed by the attack of the debris
of previous generations of animal organisms by the nitrate
ferment. By distilling a mixture of sodium or potassium
nitrate with sulphuric acid in glass vessels, or, on a large
scale, in iron vessels on which concentrated nitric acid is
PHOSPHORIC ACIDS 127
without action, nitric acid comes over into the receiver :
NaNO3 + H2SO4 = HNO3 + HNaSO4. It is not econo-
mical to use the equivalent quantity of sulphuric acid, for
the reaction between hydrogen, sodium sulphate, and sodium
nitrate takes place at a temperature so high that much of
the nitric acid is decomposed : 4HNO3 = 2H.2O + 4NO2
+ O2. Nitric acid is a colourless fuming liquid, with
very corrosive properties. In aqueous solution it is one of
the strongest acids, for it is highly ionised. It is also a
powerful oxidising agent ; this has been referred to on
p. 97. Its anhydride, N2O5, is produced on distilling
a mixture of nitric acid with phosphoric anhydride ; the
distillate separates into two layers ; the upper one consists
mainly of N2O5, and the anhydride separates in crystals
when it is cooled ; the lower layer contains H0N4On, a
liquid solidifying at - 5° ; it is the last anhydride of tetra-
nitric acid. The anhydride decomposes spontaneously in a
few days into the peroxide, N2O4, and oxygen.
Nitrates. — The nitrates are all soluble salts, possess-
ing the colours of their metallic ions. As a rule, they
crystallise easily. They are all decomposed by heat ; those
of the metals of the alkalies into nitrite and oxygen ; and
all others into the oxide of the metal and nitric oxide and
peroxide, NO a^d NO2. They are formed by dissolving
the metal, the oxide, or the carbonate in dilute nitric acid.
All metals are attacked by nitric acid, except gold and the
metals of the platinum group. The chief nitrates are those
of potassium, KNO3, saltpetre or nitre ; of sodium, NaNO3,
Chili saltpetre ; ammonium nitrate, NH4NO3, from which
nitrous oxide, N2O, is obtained on heating ; lead nitrate,
Pb(NO3)2, and silver nitrate, AgNO3, still known by its
old name " lunar caustic, " the word " lunar " referring to
the ancient alchemical connection between silver and the
moon. It is used as a caustic for removing growths and
warts ; metallic silver is deposited, blackening the place
rubbed.
Phosphates. — The source of phosphoric acid and
128 MODERN CHEMISTRY
the phosphates is chiefly calcium phosphate, Ca3(PO4)0,
a mineral known as phosphorite, and A1PO4, aluminium
phosphate, or gibbsite. Phosphoric acid is produced from
phosphorite by heating it with dilute sulphuric acid ; spar-
ingly soluble calcium phosphate is formed, while ortho-
phosphoric acid goes into solution. The solution, on
evaporation, deposits white crystals of HgPO4 ; the
residual liquor deposits crystals of H3PO4.H9O ; com-
mercial or "glacial phosphoric acid " is a mixture of both
kinds. Its solution contains many hydrogen ions, and it
is therefore a strong acid. But, inasmuch as phosphoric
4- +
acid can ionise in three ways, into 3H and PO4, into 2H
+
and HPO4, and into H and H2PO4, there are three kinds
of anions. The first of these, PO4, are present in very
small relative amount; the second and -third, HPO4 and
H9PO4, are relatively much more numerous. There is a
state of balance between the quantities of these ions present
in any solution ; and if, for example, kations of calcium or
lead or silver be added to a solution of phosphoric acid, or
to one of hydrogen di-sodium phosphate, Na.2HPO4, the
PO4 ions present enter into combination with the kations,
forming Cas(POJ,, Pb3(PO4),, or Ag3PO4 ; the PO~4
ions are increased at the expense of the HPO4 and H9PO4
ions, and the solution becomes more acid. On adding an
alkali, e.g. caustic soda, to a solution of phosphoric acid,
neutrality occurs when the salt Na0HPO4 has been reached;
+
the ions are then mainly 2Na and HPO4. On adding
more soda, the solution becomes alkaline, indicating the
presence of free OH ions ; and it is only on concentration
_ _j-
that these OH ions combine with the few H ions of the
PHOSPHATES 129
+ + ---
ionised, Na.2HPO4, forming non-ionised water, and " tri-
basic " sodium phosphate, Na3PO4, is left as a residue.
Similar remarks apply to the ortho-arsenates. The
ortho-vanadates are hydrolysed by water into pyro- and
meta-vanadates.
The chief orthophosphates are: Na2HPO4.i2H2O,
obtained by neutralising phosphoric acid with sodium car-
bonate ; HNa(NH4)PO4.4H2O, named " microcosmic
salt"; the human organism used to be known as the
" microcosm, " and this salt crystallises out of concentrated
urine ; Ca3(PO4)o, prepared by precipitation, and found native
.Ca— PO4=Ca
as phosphorite ; F— Ca— PO / , a widely
XCa-P04=Ca
spread mineral termed apatite; (NH4)MgPO4.6H2O, a
white precipitate produced by adding ammonium and
+
magnesium ions to those of a soluble phosphate : (NHJ +
Mg + PO4=(NH4)MgPO4. It is the usual test for the
presence of magnesia, and serves at the same time to dis-
tinguish phosphoric acid ; arsenates give a precisely similar
precipitate. The precipitate is nearly insoluble in am-
moniacal water, and it may be filtered off and washed with
water containing ammonia with very little loss. Like
almost all phosphates, it is soluble in water containing
hydrogen ions ; and by the addition of ammonium hydr-
oxide their number is greatly diminished. On ignition,
it yields magnesium pyrophosphate, Mg2P0(X, thus :
"
Arsenates. — The important arsenates correspond
exactly in formula and crystalline form to the phosphates ;
the only striking difference is in the colour of the silver
salt ; while ortho-phosphate of silver is yellow, the ortho-
arsenate is brick-red.
Vanadates. — Ortho-vanadates are prepared by fusing
vanadium pentoxide with the required amount of a carbonate;
130 MODERN CHEMISTRY
on addition of nitric acid to one of its salts, metavanadic
acid separates out as a brown-red powder ; on ignition, a
sparingly soluble red powder is left: 2HVO3 = H.,O +
V2O5. Ortho-antimonates are unknown.
Thio- Acids. — TMo-compounds are known, analogous
to these salts. Mention may be made of mixed oxy-thio-
phosphates, e.g. Na3POgS and Na3POS3, which result
from the action of alkaline hydroxides on phosphorus penta-
sulphide, a grey solid produced by direct combination of phos-
phorus with sulphur. They are easily decomposed by hot
water ; hence the thio-arsenates and the thio-antimonates
are better known. Arsenious sulphide, As2S3, a yellow pre-
cipitate formed by the action of hydrogen sulphide on a solu-
tion of arsenious chloride, and antimony trisulphide, Sb9S3, an
orange precipitate similarly formed, dissolve in solutions of poly-
sulphides of the alkalies; these solutions, on evaporation, deposit
crystals on cooling : As2S3 = 2KjSJ.Aq = 2K3AsS4.Aq -f-
S(n_2). Sodium thio-antimonate, Na3SbS4.9H2O, has long
been known as "Schlippe's salt." One of the sulphur atoms
may be replaced by selenium, giving Na3SbS3Se.9H0O,
thus showing the similarity in character between sulphur
and selenium.
Pyrophosphoric Acid. — When hydrogen di-sodium
orthophosphate is heated to redness, water is lost, and
tetra-sodium pyrophosphate is left as a white deliquescent
mass : 2HNa2PO4 - Na4P2Or + H2O. This salt is soluble
in water. On adding to it lead nitrate, a precipitate of
lead pyrophosphate is thrown down ; it is filtered off,
suspended in water, and a current of hydrogen sulphide
is passed through the liquid. Lead sulphide is formed,
and, on removing it by filtration, the solution contains
pyrophosphoric acid. With silver nitrate, a pyrophosphate
gives a white precipitate of silver pyrophosphate, a reaction
which distinguishes this acid from orthophosphoric acid,
for silver orthophosphate is yellow. Magnesium pyro-
phosphate has already been alluded to. The pyrophosphates
of the metals, those of the alkalies excepted, are insoluble
MET A-S ALTS 131
in water, but, for reasons similar to those given in describ-
ing the ortho phosphates, they dissolve in acids.
Pyroantimonate of potassium is a salt obtained by
fusing the metantimonate, KSbO3, with caustic potash,
and crystallisation of the resulting fused mass from water.
It is easily soluble, but on adding its solution to that of a
sodium salt, a precipitate of the sparingly soluble di-hydrogen
di-sodium pyroantimoniate, HgNagSb^CX, is produced, It
is one of the very few sparingly soluble salts of sodium.
Meta* Salts. — On heating to redness di-hydrogen
sodium orthophosphate, H9NaPO4, or microcosmic salt,
H(NH4)NaPO4.4H2O, water, or, in the latter case,
ammonia in addition, is lost, and the residue consists of
sodium hexa-metaphosphate, (NaPO3)6. It is a glass
soluble in water ; its salts are mostly gelatinous. The acid,
which is probably also hexa-metaphosphoric acid, is a
soluble glass, formed on igniting ortho-phosphoric acid ;
it yields salts like that mentioned on p. 126. Unlike the
other two phosphoric acids, it coagulates a solution of
\vhite-of-egg or albumen in water. Its silver salt is white
and gelatinous. Mono-metaphosphates are insoluble salts,
produced by igniting together oxides, carbonates, sulphates,
or nitrates with excess of phosphoric acid, and removing
the excess of phosphoric acid with water. The salts of
the alkalies are sparingly soluble. Metarsenates are pro-
duced in a similar manner to the hexa-metaphosphates, but,
on treatment with water, they combine with water and
re-form the orthoarsenates of metal and hydrogen from
which they were obtained. Some pyro- and meta-thio-
arsenates have been prepared.
Compounds containing less Oxygen. — The
elements of the nitrogen group are characterised by their
possessing more than one valency. They are also, in
most cases, capable of forming compounds with hydrogen.
These two characteristics, taken together, lead to the
possibility of their forming a number of isomeric com-
pounds, i.e. compounds having an identical composition,
132 MODERN CHEMISTRY
but being, at the same time, different chemical individuals.
Some such compounds are known, at least in their de-
rivatives. The conception will be clearer after inspection
of the following table : —
HOX /OH HO, /H HOv /H HO-. /H
HO7 NOH HO/ \OH HO/ \DH HO/
(i) (2) (3) (4)
/OH /H /H /H
\OH \OH \OH \H
(9) (10) (ii) 12)
/OH /H
(15) (16)
/OH /H /H /H
N(-OH Nr-OH N^H N^-H
\OH X)H XOH \H
(5) (6) (7) (8)
/OH /H
(13) (14)
For convenience' sake, the compounds have been written
as derivatives of nitrogen, but the type is followed by other
elements of the group.
( i ) is the true " ortho " acid, unknown in all cases.
(9) is its first anhydride, known in " orthophosphoric "
acid and in " orthoarsenic " acid. (IS) is nitric acid,
and corresponds to mono-metaphosphoric acid, metavana-
dic acid, the metarsenates, and the metantimonates. (2)
and (10) are unknown bodies, but (10) corresponds to
phosphorous acid, and (16) to the nitrites. (3) is also
unknown, but ( 1 1 ) is represented by hypophosphorous acid
and the hypophosphites. (4) is unknown. (I2)> how-
PHOSPHOROUS ACID 133
ever, may possibly be the formula of hydroxylamine ; its
name, in that case, should be " oxy-ammonia." In all
these compounds the element is a pentad. The other
compounds contain triad element. ( 5 ) probably represents
the formulae of the arsenites ; ( 1 3 ) is an alternative formula
for nitrites. (6) and (14) are unknown. (7) is an alter-
native formula for hydroxylamine.
Phosphorous Acid. — We shall first consider numbers
(8) and (16). In phosphorous trichloride, PC13, phos-
phorus is undoubtedly a triad. On heating this compound
to 60°, and passing over it a current of dry air, and subse-
quently leading the air through ice-cold water, crystals
separate, which are washed with ice- water and dried in
a vacuum. They have the formula H3PO3. The acid,
however, is di-basic ; the formula of its sodium salt, for
example, is Na.2HPO3. Again, phosphorous anhydride
P4O6, produced by the combustion of phosphorus in a
limited supply of air, constitutes a crystalline substance,
melting at 22.5° ; it is acted on only very slowly by cold
water, and then yields phosphorous acid. These facts point
to a molecular change from P(OH)3 to O=PH(OH)2.
But this view is rendered certain by consideration of the
ethyl salts of the acids.
Constitution of Phosphorous Acid. — Phosphorous
trichloride, if treated with a solution of sodium ethoxide,
Na(OC.->H-)o, in alcohol, yields tri-ethyl phosphite,
(C,H5)3PO?~ or P(OC2H5)3, corresponding to P(OH)g.
It is a liquid, boiling at 191°. On the other hand, a
compound analogous to hydrogen phosphide, PH^, is
known, of the formula PH2(C2H5), named di-ethyl
phosphine, which, on oxidation, yields a di-basic acid
analogous to phosphorous acid, O=P(C2H5)(OH)0, named
ethyl-phosphinic acid, from which the ethyl salt can be
prepared, C-P(C2H5) (OC2H5)2, isomeric with ethyl
phosphite. Ethyl phosphite is a derivative of triad phos-
phorus, whereas di-ethyl ethyl-phosphinate is derived from
pentad phosphorus. The anhydride of phosphinic acid,
134 MODERN CHEMISTRY
O
II
O=PH(OH)2, would be, not P2O3, but H-P=O, like
(16). But this substance is not formed on heating phos-
phorous acid, for it decomposes into phosphoric acid and
phosphoretted hydrogen, thus: 4HgPO3 = PHg + 3H3PO4.
One of the varieties of the salts of nitrous acid, however,
has a corresponding formula.
Nitrites. — When a nitrate of one of the metals of the
alkalies is heated with metallic lead, lead monoxide is
formed, and a nitrite, thus : KNO3 + Pb = PbO + KNO2.
The nitrite is left as a white fusible salt, easily soluble in water.
On acidifying a very dilute solution with sulphuric acid, a
dilute solution of nitrous acid is formed ; but on warming it,
a somewhat complex action takes place. First, the anhy-
dride is produced : 2HNO9.Aq = H9O.Aq + N9O3 ; next,
the anhydride is attacked by the water and decomposed :
3N^O3 + H.2O = 2HNO3 + 4NO ; and some of the anhy-
dride volatilises with decomposition into nitric oxide and
peroxide : N0O3 = NO + NO9. The nitrites are white,
easily soluble crystalline salts ; those of lead, Pb(NO9)9,
and silver, AgNO9, are sparingly soluble. All are decom-
posed by the stronger acids ; for nitrous acid is a weak acid,
and, moreover, it is easily decomposed, as has been pointed
out. None of these changes throws any light on the constitu-
tion of nitrous acid, however. To gain this knowledge it is
necessary to study the alkyl salts ; for example, ethyl nitrite.
Constitution of the Nitrites. — Ethyl nitrite,
made by distilling together a mixture of sodium nitrite,
sulphuric acid, and alcohol, is a volatile colourless liquid
with a fragrant odour. On boiling it with a solution of
sodium hydroxide it is hydrolysed, the ethyl group being
again replaced by the metal sodium, thus : O=N— O(C9H5)
+ NaOH.Aq = O=N-ONa.Aq + C2H5OH. And if
ethyl nitrite be placed in a flask along with tin and hydro-
chloric acid — in other words, exposed to the action of
nascent hydrogen — the products are ammonia (with some
NITRITES 135
H H H
hvdroxylamine) and alcohol : O = N-HorO + N-H +
HO(C.,H5). Sodium nitrite therefore appears to possess
the formula O=N— ONa. But silver nitrite, heated in a
sealed tube with ethyl iodide, yields a compound of the
same composition as, but not identical with, ethyl nitrite,
O
II
and the formula N— (C.2H5) is ascribed to it ; for, on heat-
II
O
ing with caustic soda, it is not hydrolysed, but one of the
atoms of hydrogen of the ethyl group is replaced by the
O
II •
element sodium, giving N— (C.?H4Na) ; and further, with
II
O
nascent hydrogen, the two atoms of oxygen are removed
and replaced by hydrogen, yielding ethylamine, a compound
H
analogous to ammonia, N— (C0H5) ; this shows that the
H
ethyl group in the compound, which is named nitroethane,
is in direct union with the nitrogen atom. It appears, then,
O
II
that silver nitrite has the formula N— Ag, and not
6
O=N— OAg. It also follows that two nitrous acids
O
il
must exist, O=N— OH and N— H, the former (13) a
II
O
136 MODERN CHEMISTRY
derivative of triad, and the latter (16) of pentad nitrogen.
But the acids are unknown, and it is only possible to guess
the constitutional formulae of the salts through the reactions
just described.
Arsenites. — Arsenites, derived from the acid
HgAsOg, such as hydrogen cupric arsenite or " Scheele's
green," HCuAsO3, produced by adding to a solution of
copper sulphate potassium arsenite, arsenious oxide, and a
little ammonia ; pyro-arsenites, such as K4As9O5, and
metarsenites, KAsO9 ; also ortho- and meta-thioarsenites,
K3AsSg and KAsS9, are known. They show no signs of
isomerism like that of the phosphites and nitrites, and they
are doubtless salts of As(OH)3 and O=As— OH, and
the corresponding sulphur acids, although the acids corre-
sponding to the sulphur salts are unknown. Metantimonite
of sodium, NaSbO9, and metarthioantimonite, NaSbS9, are
formed by dissolving antimonious oxide, Sb4O6, or the sul-
phide, Sb9S3, in caustic soda, and precipitating with alcohol.
Hypophosphites. — Hypophosphorous acid, H3PO2,
is a monobasic acid ; sodium hypophosphite has the formula
Na(H9PO9). This leads to a formula analogous to that
given in ( 1 1 ) . When heated, too, the acid yields phosphine
and phosphoric acid: 2HgPO2 = PH3 + HgPO4. This
would lead to the supposition that some of the hydrogen
was already in combination with the phosphorus. Its salts
also yield phosphine, together with a phosphate and free
hydrogen. The acid is prepared by the action of sulphuric
acid on the barium salt ; that salt is prepared by boiling
together yellow phosphorus and caustic baryta : 2P4
+ 3Ba(OH)9.Aq + 6H2O = 2PH3 + 3Ba(H2PO2)2.Aq.
With sulphuric acid insoluble barium sulphate is formed,
while hypophosphorous acid remains in solution. It forms
white crystals, melting at 17.4°. The acid has reducing
power ; with silver nitrate, for example, metallic silver is
precipitated and phosphoric acid is formed. With hydrogen
iodide phosphorous acid and phosphonium iodide are formed :
3H(H2P02) + HI = 2H2(HP03) + PH4I.
HYPOPHOSPHATES 137
Two acids are known belonging to this group of ele-
ments ; they have not been tabulated on p. 132, because
their structure may be compared with that of hydrazine or
liquid phosphine, H2N— NH2 or HaP— PH2, in which two
atoms of nitrogen — or of phosphorus — are in direct union
with each other. These are phosphatic acid, or, as it is
0=P=(OH)2
sometimes termed, hypophosphoric acid, I ,
0=P=(OH)2
N-(OH)
and hyponitrous acid, II . The first of these is
N-(OH)
produced in small quantity along with ortho-phosphoric and
phosphorous acids, when phosphorus is oxidised by exposure
to moist air. It is, however, best made from its silver salt,
by addition of the equivalent quantity of hydrochloric acid.
Silver hypophosphate is produced by dissolving 6 grams of
silver in 100 grams of nitric acid diluted with its own
weight of water, and adding to the solution, warmed on a
waterbath, 8 or 9 grams of phosphorus. As soon as the
violent evolution of nitrous gases ceases the liquid is cooled,
and silver hypophosphate crystallises out. The acid has
no reducing properties, hence it probably contains no hydro-
gen capable of conversion into hydroxyl by the addition of
oxygen. The sodium salt, Na4P2O6, is converted into
pyrophosphate by the action of a solution of bromine in
water ; the change is evidently due to the addition of
"
oxygen, thus : + O = O . The
0=P=(OH)2
anhydride of this acid would be ; a compound of
0=P=0
the formula P2O4 is produced by the incomplete combustion
of phosphorus in oxygen ; but as it yields orthophosphoric
138 MODERN CHEMISTRY
and phosphorous acids on treatment with water, it is in all
O
probability phosphoryl phosphate, O=P~O~P\\ .
X^o
Hyponitrites are produced by the action of sodium
amalgam, that is, a solution of sodium in mercury contain-
ing about 4 per cent, of the former, on a solution of potas-
sium or sodium nitrite. After the mixture has stood for
some days, it is rendered slightly acid with acetic acid, and
silver nitrate is added. A yellow precipitate of silver
hyponitrite is produced ; other hyponitrites may be pre-
pared from it by the addition of the calculated quantity of
the respective chloride. The acid can also be liberated by
the addition to a very dilute aqueous solution of the equiva-
lent amount of hydrochloric acid. On warming the solution
of the acid, nitrous oxide is evolved ; but nitrous oxide
does not unite with water to form the acid.
That the acid has the formula HL,N2O0, and not HNO,
is shown by its formation from hydroxylamine and nitrous
acid. On mixing dilute solutions of hydroxylamine sulphate
and sodium nitrate, the hydroxylamine nitrate loses water,
thus: HO-NH2 -i- O=N-OH = H2O + HO-N=N-OH;
the silver salt is thrown down on addition of silver nitrate.
CHAPTER VIII
The Oxy= Acids of the Halogens ; Perchlorates and
Periodates ; Chlorates, Bromatest and lodates;
Chlorites ; Hypochlorites, Hypobromites, and
Hypoiodites— Acids and Salts of Sulphur, Sele-
nium, and Tellurium ; of Molybdenium, Tung*
sten, and Uranium — Perchromates, Persulphates,
Perborates, and Percarbonates.
THE formulae of the acids of the halogens present some
analogy with those of the nitrogen group, for, like the
latter, the halogens also possess uneven valency. But while
the highest valency of elements of the nitrogen group is
that of a pentad, chlorine and iodine function as heptads in
perchloric and periodic acids. The valency of the halogens
is five in chloric, bromic, and iodic acids ; three in chlorous
acid ; and one in the hypochlorites, hypobromites, and
hypoiodites. A short table, analogous to that given on
p. 132, shows the relation between these compounds : —
I(OH)6(ONa), corresponding to I(OH)T, ortho-
periodates ;
O=I(OAg)5, corresponding to O=I(OH)5, para-
periodates ;
°\ °\
/^I(OAg)3, corresponding to \I(OH)g, meso-
( middle) periodates ;
139
MODERN CHEMIST'RY
°\
O^I(OAg), corresponding to O^I(OH), meta-
periodates.
^°
~-=rI— O— Is=O, unknown periodic anhydride.
Perchloric acid, O3C1(OH), corresponding to meta-
periodic acid, is the only representative of these among
the other members of the halogen group. But the
periodates, like the phosphates, form still more condensed
acids ; thus, salts of a di-ortho-periodic acid, H8I2O1;L,
and of a di-meso-periodic acid, H4I2O9, as well as of a
tetra- and hexa-periodic acid, H10I4O19 and H0()I6O01,
are known.
T(OH)5, ortho iodic acid; orthobromic and ortho
chloric acids are unknown.
O=I(OH)3, and similar bromine and chlorine acids, are
unknown.
O^ O^ O^
J^I(OH), iBr(OH), and ^Cl(OH), iodic,
bromic, and chloric acids.
°\ SQ
\\ — O — 1\\ , iodic anhydride, is the only an-
O^ ^O
hydride known.
A tri-iodic acid, HI3Og, has been prepared.
O=C1— (OH), chlorous acid, is the only representative
of triad halide.
I-(ONa), Br-(ONa), and Cl-(ONa), hypoiodite,
-chlorite, and -bromite of sodium and of some other
metals are fairly stable in solution.
//ypoc/i/or/tes. — The starting-point for these com-
pounds is the hypohalite ; it is produced by the action of a
hydroxide on the element in cold aqueous solution, thus :
HYPOCHLORITES * 141
2NaOH.Aq + Cl.>, Br0, or I, = NaCl, NaBr, or Nal.Aq
+ H,O + NaOCl," NaOBr, or NaOI. Aq.
Chlorine monoxide, C19O, is formed on passing
over dry mercuric oxide, prepared by precipitation from
mercuric chloride with caustic soda, dry chlorine gas ; the
tube containing the oxide must be cooled with ice, for the
monoxide is a dark brown, very unstable liquid, boiling
at 6°. The equation is: 2HgO + 2C12 = Hg2Cl2O +
Cl— O— Cl. Its density at 10° corresponds with the formula
given. If the mercuric oxide be made into an emulsion
with water, and chlorine be passed through, the acid is
produced in aqueous solution ; it is a pale yellow liquid,
with a not unpleasant smell, recalling that of chlorine. If
concentrated, it decomposes into water, chlorine, and oxygen.
It reacts at once with hydrochloric acid, forming water and
chlorine : H-O-C1. Aq + H-Cl.Aq - C12 + H9O.Aq.
The most important hypochlorite is a double compound,
obtained by the action of chlorine on slaked lime, termed
"chloride of lime" or " bleaching-powder." It is a
white, non-crystalline powder, smelling of hypochlorous acid.
Its formula is Cl— Ca— O— Cl. That it is a compound, and
not a mixture of calcium chloride and hypochlorite, is
proved by the fact that bleaching-powder is not deliquescent,
whereas calcium chloride is a very deliquescent salt; calcium
chloride and hypochlorite are both very soluble salts, but
bleaching-powder is only sparingly soluble, but if a saturated
solution of bleaching-powder be cooled, crystals of hypo-
chlorite separate out, thus proving that it is dissociated in
aqueous solution into these two salts. Its smell, as well as
that of other hypochlorites, is due to the fact that hypo-
chlorous acid is a very feeble acid, and is only slightly
ionised ; hence the calcium and other salts are hydrolysed
by the ions of water, and the solution contains free base
and free acid ; and the latter reveals its presence by its smell.
No ion has a smell ; hence one does not smell solutions of
salts, but only volatile non-ionised compounds.
When bleaching-powder is distilled with just enough
i42 MODERN CHEMISTRY
acid to liberate the hypochlorous acid, that acid comes over ;
but if excess of such an acid as sulphuric or hydrochloric
be added, chlorine is liberated, owing to the reaction
between hydrochloric and hypochlorous acids. The addition
of a trace of a salt of cobalt to bleaching-powder results in
the liberation of oxygen when it is warmed ; this reaction,
which is termed " catalytic," is supposed to be due to the
alternate formation and decomposition of an oxide of cobalt
of the formula Co4O7 ; but the reaction is still obscure.
The bleaching action of bleaching-powder in presence of
acid is ascribed to the liberation of oxygen, and the oxida-
tion thereby of the insoluble brownish colouring matter of
unbleached cotton or linen to soluble compounds which can
be removed by washing.
Chlorates. — Hypochlorites, when heated, undergo
conversion into a mixture of chlorate and chloride :
3NaOCl.Aq = NaClO3.Aq + 2NaCl.Aq. The usual
method of preparing chlorates, however, is to pass a current
of chlorine through hot "milk of lime" — that is, calcium
hydroxide suspended and partially dissolved in water.
Potassium chloride equivalent to one-sixth of the lime is
also present. The following reaction occurs: 6Ca(OH)2.Aq
+ 2KC1. Aq + 6C12 = 6CaCl2. Aq + 2KClOg. Aq + 6H2O.
On evaporation, the sparingly soluble potassium chlorate de-
posits in crystals, leaving the very soluble calcium chloride in
solution. The potassium chlorate is purified by recrystallisa-
tion. It is a white, lustrous salt, crystallising in flat plates.
It fuses readily, and, at a somewhat higher temperature than
its melting-point, it evolves oxygen. At the same time some
of the chlorate is oxidised by the oxygen, and perchlorate
is formed: 2KC1(X = 2KC1 + 30,, and KCKX + O =
KC104.
Perchlorates. — On ceasing to apply heat, therefore,
after the salt has become pasty, and treating with water,
the potassium chloride is dissolved, leaving the much less
soluble perchlorate ; the perchlorate may be purified by
recrystallisation.
CHLORINE PEROXIDE 143
Owing to the fact that very few potassium salts are
insoluble in water, it is not convenient to prepare chloric
acid from the potassium salt ; for this purpose it is
better to use the barium salt, made from baryta-water
and chlorine ; a solution of this salt, when mixed with
the equivalent amount of dilute sulphuric acid, yields a
precipitate of barium sulphate, and chloric acid remains in
solution.
Chlorine Peroxide. — The solution, freed from barium
by filtration, may be concentrated by distilling away the
water at a low temperature in a vacuum ; the acid remains
as a colourless, syrupy liquid, which decomposes at 100°
into perchloric acid, water and chlorine peroxide, CIO., ;
the last compound is unstable at that temperature, and
explodes into chlorine and oxygen. But the peroxide may
be prepared by warming, not above 40°, a mixture of potas-
sium chlorate and concentrated sulphuric acid ; the chloric
acid decomposes as it is formed: 3HC1O3 = HC1O4 +
H2O + 2C1O2. It is a dark red liquid, boiling at 10.6°
to a reddish-brown gas. Unlike nitric peroxide, it does
not form double molecules ; C19O4 is unknown. It re-
sembles that compound, however, in its action on water ;
while nitric peroxide gives a mixture of nitrous and nitric
acids, chlorine peroxide, added to an alkali, forms a chlorite
and a chlorate : 2C1O2 + iKOH.Aq = KClO9.Aq +
KClO3.Aq.
The chlorates, like the nitrates, are all readily soluble
in water ; lead and silver chlorites, like the corresponding
nitrites, are sparingly soluble salts ; and lead perchlorate is
the only salt which does not easily dissolve. As already
mentioned, chloric acid is readily decomposed when its
aqueous solution is warmed ; chlorous acid is still less
stable ; but perchloric acid, which may be prepared by
distilling together potassium perchlorate with concentrated
sulphuric acid, is relatively stable, seeing that it can be
distilled without decomposition. It is an oily liquid, with
acid taste ; it is apt to explode when brought into contact
144 MODERN CHEMISTRY
with any oxidisable matter. The corresponding perbromic
acid is unknown.
Hypobromites. — Hypobromites are produced, along
with bromides, on mixing solutions of alkalies with bromine ;
the solution turns yellow, and acquires a smell like that of
seaweed. On warming, a change analogous to that suffered
by hypochlorites occurs ; the hypobromite yields bromide
and bromate, and the latter can be separated by crystal-
lisation.
Bromates. — The bromates are white salts soluble in
water ; they do not, however, decompose into bromide and
perbromate when heated ; the perbromate is unstable, and
bromide and oxygen are the only products. Bromic acid,
too, when warmed changes to water, hydrobromic acid,
bromine, and oxygen ; as no compound analogous to C1O2
is produced, bromous acid is unknown.
Hypoiodites. — The formation of hypoiodites is
analogous to that of hypochlorites ; but the salts are
known only in solution mixed with iodide. Again, like
the hypochlorites, they change on heating ; they yield a
mixture of iodide and iodate ; and from barium iodate
iodic acid can be prepared. But it is more readily obtained
by boiling iodine with nitric acid ; for iodine is more easily
oxidised than either chlorine or bromine ; or chlorine and
water may be used as an oxidising agent.
Iodic Acid. — Iodic acid is a white crystalline com-
pound, easily soluble in water ; it is a strong acid, and its
salts are produced by neutralisation with hydroxides or
carbonates. When it is mixed in solution with hydriodic
acid, mutual decomposition ensues and iodine is liberated :
HI03. Aq + sHI. Aq = 3 12 + 3H20. Aq.
Periodic Acid. — The oxidation of iodic acid to
periodic acid is accomplished by means of a solution of
sodium hypochlorite ; it is easier to dissolve iodine in a
solution of sodium carbonate, when hypoiodite is formed,
and to saturate the solution with chlorine. The iodate at
first formed is converted into the periodate : NaIOg.Aq-f-
THERMAL DATA 145
NaOCl. Aq = NaCl. Aq + NaIO4< Aq. As the periodate is
sparingly soluble in water, it crystallises out on concentrat-
ing the solution. On mixing the solution of the sodium
salt with silver nitrate, tri-hydrogen di-argentic periodate
is precipitated ; it is dissolved in hot dilute nitric acid and
evaporated, when mono-argentic periodate, AgIO4, crystal-
lises out. On mixing with water, this salt undergoes the
change : 2AgIO4 + 4H?O = H8Ag2IO6 + H5IO6.Aq.
The silver salt, which is insoluble in water, is removed by
filtration, and the periodic acid deposits in crystals on
evaporation. The acid forms white prisms ; on heating it
to 130°, it decomposes into iodine pentoxide, I2O5, a
white solid, also produced on heating iodic acid to 170°,
together with water and oxygen ; at 1 80° the pentoxide
decomposes slowly into iodine and oxygen.
Thermal Data. — From the short description which
has been given, it is seen that the oxides of iodine and their
compounds are, as a rule, more stable than those of bromine
and chlorine, and this is connected with the heat which is
evolved or absorbed during their formation. This heat is
seldom determined directly ; never when the compounds
are produced with absorption of heat. Thus, when chlorine
combines with oxygen to form C1.,O, enough heat is ab-
sorbed to cool 17,800 grams of water through i°, or what
is the same thing, on decomposing C12O heat enough is
liberated to raise the temperature of 17,800 grams of
water through i°. This is termed the heat of formation
of the substance. The heat of formation of chloric
acid from chlorine, oxygen, and water involves a heat-
absorption of 20,400 calories, and these substances are
both very unstable. On the other hand, the combination
of iodine with oxygen is attended with an evolution of
heat of 25,300 calories, and an additional 2600 calories
are liberated when it combines with water to form iodic
acid. Perchloric acid, too, is formed with evolution of
heat (4200 calories), and thus iodic, periodic, and per-
chloric acid are comparatively stable. The heat-change
146 MODERN CHEMISTRY
during the formation of a compound, therefore, is connected
with its stability, although the exact relationship between
the two is at present unknown.
Acids derived from Elements of Even Valency.
— Elements of the molybdenum and of the sulphur
groups can act as dyads, tetrads, and hexads, and there are
corresponding compounds of chromium, manganese, and
iron, while sulphur and manganese are also able to form
compounds analogous in formula to the perchlorates, termed
the permanganates and persulphates. Compounds in
which these elements function as dyads, however, have no
acid properties ; in the case of chromium, manganese, and
iron, the dyad oxides have more or less basic properties —
that is, their hydroxides are ionised into element and
hydroxyl — and they therefore form salts with acids. The
oxides MoO2 and UO2 are also feebly basic in character,
as well as TeO9 ; but MnO2 is the anhydride of a feeble
acid, and SO2 and SeO9 form well-defined acids, sulphu-
rous, H2SO3, and selenious, H2SeOg.
Oxides of Sulphur, Selenium, and Tellurium.
The corresponding Acids. — When sulphur, selenium,
or tellurium is heated in air, the element takes fire and burns ;
the chief product in each case is the dioxide. That of
sulphur is a colourless gas, possessing the well-known odour
of burning sulphur ; it is condensable to a liquid at -8°, and
it freezes at -79°. The gas is soluble in water, uniting
with it to form the acid, H2SO3 ; on cooling the solution
to -6° and saturating it with the gas, crystals of the formula
H.^SOg.SH^O separate; this is a hydrate of sulphurous
acid. Selenious anhydride, a white solid, also dissolves in
water, and from the solution selenious acid crystallises
out, with the formula H2SeO3. It would naturally be
imagined that these acids should have the structural formulas
OS=(OH)2 and O=Se(OH)2, inasmuch as the oxides
are O=S=O and O=Se=O ; moreover, the chlorides
O=S=C12 and O=Se=Cl;, thionyl and selenosyl chlo-
rides, are known ; and these react at once with water,
ISOMERIC SULPHITES 147
forming the acids. It is to be presumed that there is ex-
change of chlorine for hydroxyl, as usual : O=S=C1., +
2HOH = O=S=(OH)2 + 2HC1. But there is evi-
dence, similar in kind to that adduced in the case of nitrous
and phosphorous acids, to show that while sodium sulphite
has the formula O=S=(ONa)9, silver sulphite is better
expressed by /^S\ > sulphur being a hexad.
O^ X)Ag
Isomeric Sulphites. — The evidence is this : — Sul-
phur alcohol or ethyl-hydrosulphide (also termed " mer-
captan"), when oxidised by boiling with dilute nitric
acid, is converted into ethyl-sulphonic acid, thus :
O
C9H5SH + 3O = C2H-S-OH, a monobasic acid, of
li
O
O
I!
which the ethyl salt is C0H5— S— O— C9H5. Now sodium
6
sulphite, warmed with ethyl iodide, yields an isomeric com-
O
pound of the formula C.,H5-O-S-O-C2H5. This is
known, because when saponified by boiling with alkali, it is con-
O
li
verted into alcohol and a suluhite, thus: C0H-O— S— OC,H,
/OK
+ 2KOH=2C2H5OH + O=S<' ; whereas the sap-
XOK
oniiication of ethyl sulphonate yields potassium ethyl-
O
sulphonate and alcohol, thus : C2H5— S— O— C2H5 +
II
O
i48 MODERN CHEMISTRY
O
KOH = C9H5-S-OK + C,H5OH. And, moreover, this
II
O
acid, when distilled with phosphoric chloride, yields an
O
II
acid chloride, C0H5— S— Cl, which can be reduced with
II
O
nascent hydrogen to ethyl hydrosulphide, the substance from
which the acid was originally obtained by oxidation. It is
therefore concluded that the carbon is directly united to the
sulphur atom in this case, while in ethyl sulphite the carbon
of the ethyl group is united through oxygen. It follows
/OH
that sulphurous acid must have the formula O=S<^ ,
\OH
Osv /H
whereas sulphonic acid should be represented by ^S\
O^ \OH
The silver salt is a sulphonate, while the potassium salt is
a sulphite. This peculiarity is not shown by selenium or
tellurium. It appears certain that they are represented by
the formulae O=Se^ and O=Te<^ ; but it is
\OH \OH
not known which formula is to be ascribed to a solution
of sulphur dioxide in water.
Sulphites. — The sulphites, selenites, and telluritesof
the alkalies are soluble salts; those of most of the other metals
are sparingly soluble in water. Double salts with hydrogen
(" acid salts ") are, however, soluble, e.g. calcium hydrogen
sulphite, Ca(HSO3)9 ; and they are all decomposed by the
stronger acids, sulphurous acid being liberated, if the solution
is dilute ; if strong, sulphur dioxide, its anhydride, comes
off in the state of gas. Similarly, selenious and tellurous
ACID CHLORIDES 149
acids are liberated on addition of a strong acid to a solution
of a selenite or tellurite. Pyro sulphites, similar in kind to
o o
\( II
pyrophosphates, such as KO— S— O— S— OK, crystallise
out on passing a current of sulphur dioxide through a
solution of the carbonate of the alkali.
Sulphurous acid is a reducing agent, depriving reducible
compounds of their oxygen ; it itself is oxidised to sulphuric
acid by the process. Owing to this property, it is used to
bleach woollen goods ; this it does by converting the in-
soluble colouring matter into a soluble colourless compound,
which can be removed by washing. It is also an antiseptic ;
and sulphites are added to liquors undergoing fermentation,
when it is desired to check the action of the ferment.
Selenious and tellurous acids, treated in boiling solution
with sulphurous acid, deposit selenium or tellurium, thus :
H2SeO3.Aq + 2H2SO3.Aq = Se + 2H,SO4.Aq + H2O ;
and with sulphuretted hydrogen, sulphurous acid gives a
precipitate of sulphur : H2SO3.Aq -f- 2H9S.Aq =38 +
2H.>O-Aq. This brings to mind the mutual action of
hydrochloric and hypochlorous acids, and of hydriodic and
iodic acids, where the elemens are also liberated.
Acid Chlorides. — Sulphur dioxide combines with
chlorine when a mixture of the two gases is exposed to
sunlight, or when it is passed over gently heated charcoal.
CK Cl
The product, sulphuryl chloride, ^>S<; , is a colourless
O^ \C1
fuming liquid, boiling at 7 7°. On adding it to water, it imme-
diately yields sulphuric acid by replacement of the chlorine
O.x 7C\ H— OH O.x /OH HC1
byhydroxyl: ^S< + = J$/ + .
O^ \C1 H— OH O^ \OH HC1
Selenium and tellurium form similar compounds ; and so
also do molybdenum, tungsten, and uranium, as well as
chromium. Molybdyl, tungstyl, a-nd uranyl chlorides
150 MODERN CHEMISTRY
are produced by passing chlorine over the dioxides heated
to redness ; they are not decomposed by water, but when
boiled with alkalies they are converted into molybdates,
tungstates, or uranates. Chromyl chloride, on the other
hand, is formed by distilling together a chromate, a chloride,
and concentrated sulphuric acid. This amounts to the
action of hydrogen chloride on chromium trioxide, thus :
CrO3 + 2HC1 = CrO0Cl0 + H.,O. The presence of the
sulphuric acid is necessary in order to withdraw and retain
water, for chromyl chloride is at once attacked by water,
chromic acid being formed. It is a deep red fuming liquid,
hardly distinguishable from bromine in appearance ; it boils
at 1 1 8°. A manganyl chloride is said also to have been
prepared.
The constitution of the acids is inferred from that of the
chlorides ; and in the case of chromium, an intermediate
body is known between chromyl chloride and potas-
sium chromate, termed chlorochromate ; its formula is
O Cl
\\ /
^vCr<( ; with sulphur, the corresponding acid,
O^ X)K
chlorosulphuric, or, better, chlorosulphonic acid is known,
CK Cl
/ S<^ . These bodies are produced by the method of
O^ X)H
mixture; the former by crystallising together anhydro-chro-
mate and chloride of potassium: j^Crr"" ^*/Cr\
o^ NOK KO/ V)
O^ /OK O^ /OK
+ KCI = ^Cr/ + >Cr< ; the latter, by the
O^ \OK O" \C1
union of hydrochloric acid with sulphur trioxide, thus :
°N\ °^ /°H
J;S = O + HC1 = >S<( . The former consists of
O^ O^ \C1
red crystals ; the latter is a fuming liquid, readily acted on
CHROMATES 151
by water, with formation of sulphuric and hydrochloric acids.
We have thus with sulphur and with chromium the series :
Cl Osv OH O. OH
\ \
XC1 0<^ \OH
Cl O. OK O OK
o^ NCI' o^ \ci o^ NOR
Chromates. — The starting-point for the chromates is
chrome iron ore, Fe(CrO2)2, a spinel (see p. 100). It
is heated in a powdered state with a mixture of lime and
potassium carbonate, in a reverberatory furnace, where the
atmosphere is a strongly oxidising one. The product is a
mixture of calcium and potassium chromates and ferric
oxide : 2Fe(CrO2)2 + 4K2CO3 + yO = Fe2O3 + 4K2CrO4
4- 4CO2. The fritted mass is treated with water, when
the chromate dissolves, leaving the ferric oxide insoluble.
On evaporation, potassium chromate crystallises out. If
it is desired to produce " bichromate " or anhydrochro-
mate of potassium, K2Cr2O7, the solution of the chromate
is treated with dilute sulphuric acid ; calcium sulphate is
precipitated, and is removed by settling ; on evaporation,
sparingly soluble sulphate of potassium crystallises c>ut ; and
after removal of the crystals, on further evaporation, " bi-
chrome " crystallises. The conversion of the chromate into
the anhydrochromate is represented by the equation :
2K2CrO4. Aq + H2SO4.Aq = K2Cr2Or.Aq + K2SO4. Aq.
This conversion is accompanied by a colour-change ; for
the ions of chromate, CrO4, are yellow, whereas those of
anhydrochromate, Cr2O7, are orange. On addition of
potassium hydroxide to the bichromate, the opposite change
takes place ; the anhydro-chromate ion is changed into the
+-- +- +--
chromate ion : K2Cr2Or Aq + 2KOH.Aq = 2K2CrO4.Aq
+ H2O.
152 MODERN CHEMISTRY
Chromic Acid. — Chromic acid is liberated on add-
ing to a concentrated solution of potassium anhydrochro-
mate a sufficient excess of sulphuric acid : K9Cr2O7. Aq +
H,SO4 = K2SO4. Aq + H2O + 2CrO3. The acid, in con-
centrated solution, loses water, and deposits the trioxide or
anhydride in crystals of a deep red colour. Chromium
trioxide is a powerful oxidising agent ; hence it may not be
brought into contact with filter-paper ; it must be filtered
through a mat of asbestos or glass wool. The excess of
sulphuric acid and potassium sulphate are washed out with
concentrated nitric acid, in which the anhydride is almost
insoluble ; the nitric acid is then volatilised by gentle heat.
This anhydride dissolves in water, but it is doubtful whether
the acid H0CrO4 is contained in the solution ; it is more
+ +
probable that the ions are HH and Cr^CX, from the colour,
and other tests, such as the conductivity.
Oxidation by means of a solution of chromic anhydride
is carried out either by boiling the substance to be oxidised
with a mixture of bichrome and dilute sulphuric acid, or
with a solution of chromic anhydride in pure acetic acid ;
the chromate ion, CrO4 or Crk>On, is changed into the
+ + + + - -
chromic ion Cr ; the action is: K9Cr2O7.Aq +
+ - - + + + - - + - -
4H2SO4.Aq = Cr2(SO4)3.Aq + K,SO4.Aq + 4H2O +
30. If the sulphuric acid is hot and concentrated, oxygen
is evolved as gas ; if dilute, substances present in solution,
if they are capable of being oxidised, are attacked by the
oxygen. When chromic anhydride is heated, it is con-
verted into chromium sesquioxide, Cr9O3, with evolution
of oxygen.
Manganates. — Oxides of manganese, if heated with
caustic alkalies in a current of air, or with potassium or
sodium nitrate, are converted into manganate ; the manga-
nate, however, is much more easily decomposed than the
chromate, and, indeed, is stable only in presence of excess
PERMANGANATES 153
of alkali. Manganic acid is incapable of existence ; an
attempt to liberate it, by addition of an acid to its sodium
salt, results in the formation of a permanganate and a
manganous salt, thus: 5Na.?MnO4.Aq + 6H2SO4.Aq =
5Na2SO4.Aq + MnSO4.Aq + 4HMnO4.Aq + 4H2O.
Permanganates. — While the manganates are bright
green, the perm? ^ .nates, which are analogous to the per-
chlorates, are almost black ; they dissolve in water with a
deep purple colour ; the best known is the potassium salt,
a solution of which is sold under the name of " Condy's
Fluid." It is also a useful oxidising agent. If an oxidis-
able body is boiled with its solution, it loses oxygen, thus :
2KMnO4.Aq + 3H2O = 2KOH.Aq + 2MnO(OH)2
+ 3O ; if an acid, such as sulphuric acid, is present, the
equation is : 2KMnO4.Aq + 3H2SO4.Aq = K2SO4. Aq +
2MnSO4.Aq + 50 + 3H2O.
Ferrates are also known ; they are still more unstable
than manganates.
Equations Simplified. — A word may be added
here with regard to the somewhat complicated equations
such as those given. It is convenient to assume the exist-
ence of the anhydride of the acid as a constituent of the
salt ; thus potassium bichromate may for this purpose be
regarded as consisting of K2O in union with 2CrO3. On
acting on it with sulphuric acid in presence of an oxidisable
compound, the K.,O may be supposed to react with the acid
thus: K2O+H2SO4 = K2SO4 + H2O. The chromium
salt formed may be regarded (and this was formerly the point
of view) as a compound of 3SOg, the anhydride of sulphuric
acid, with Cr2O3, viz., Cr2O3.3SO3, or Cr2(SO4)3. The
formation of Cr2O3 from 2CrO8 involves the loss of 30 ;
hence the equation given above. Similarly, the oxidising
action of potassium permanganate may be formulated thus :
K2O.Mn2O7 = K2O + 2MnO2 + 30 ; and K2O.Mn2O7
= K2O + 2MnO + 50. With water present in the former
action, the K2O becomes KOH, and the manganese dioxide
becomes hydra ted ; with sulphuric acid present in the latter,
154 MODERN CHEMISTRY
the K2O is converted into K.,SO4, and the MnO into
MnSO4. This old method of representing chemical
changes had much to recommend it on the score of simpli-
city ; and it often is found convenient, although it is only
a partial expression of the truth.
Molybdates, Tungstates, and Uranates. — The
formulas of the molybdates, tungstates, and uranates
are analogous to those of the chromates ; for example,
K2Mo04, Na2W04, (NH4)2UO4. The common ore of
molybdenum is the disulphide, crystalline scales resembling
graphite, MoSt>, termed molybdenite. On heating it in
the air, or on boiling it with concentrated nitric acid, it is
oxidised to the trioxide, MoO3, a white slippery powder.
Wolfram, (Fe,Mn)WO4, is the chief ore of tungsten ; on
boiling with concentrated nitro-hydrochloric acid, calcium
nitrate and chloride go into solution, and tungstic acid,
HoWO4, remains as an insoluble yellow powder. On
heating it, it loses water, and yields the anhydride, a
powder with similar colour, WO3. Pitchblende is the
name of the commonest ore of uranium ; its formula is
UgOg. On fusing it with a mixture of nitrate and car-
bonate of soda, sodium uranate NaQUO4 is formed ; and on
adding acid, uranic acid, H9UO4 is precipitated, as a
yellow powder. On heating it to 300°, a scarlet powder,
of the formula UO3, remains. Ignition changes it into
U3O8, possibly uranium uranate, U(UO4)2, of the same
formula as the natural mineral. The chief molybdate is
that of ammonium, (NH4)9MoO4, white crystals obtained
by dissolving the acid in ammonia solution ; it is used in
precipitating phosphoric acid as phospho- molybdate of
ammonium, a representative of many very complicated molyb-
dates ; its formula is i6MoO3.P2O5.3(NH4)2O.i4H2O ;
it is a derivative of one of the condensed molybdic acids.
Sodium tungstate, NazWO4, produced by fusing the tri-
oxide with sodium carbonate, is used as a mordant in dye-
ing, and it has the property of rendering cotton and linen
fabrics uninflammable. The chief characteristic of uranium
SULPHUR TRIOXIDE 155
trioxide is that of forming uranyl salts, such as uranyl
nitrate, (UO0)(NO3)9, and acetate, (UO.,) (C.,H3O0)0,
"O
where uranyl, U = acts as a dyad radical. The uranates
il
O
are ill-defined compounds.
Sulphur Trioxide. — The constitution of sulphuryl
chloride and its conversion into sulphuric acid has already
been alluded to. And it may be assumed that that of sulphur
//Q
trioxide, SO3, is expressed by the formula 0=8^ > sulphur
acting as a hexad. Although sulphur dioxide unites directly
with chlorine, it does not combine with oxygen, unless the
two gases are brought intimately into contact by passing them
over finely divided platinum ; such platinum is best prepared
by dipping asbestos (a native magnesium silicate, possessing
a fibrous structure) into platinic chloride, and subsequent
ignition, when the chloride is decomposed into chlorine,
which escapes, and a deposit of spongy platinum on the
asbestos. On a large scale, sulphur dioxide, made by
burning sulphur or iron pyrites, FeS2, in air, is concentrated
by solution in water, the gas being forced in under some
pressure ; the solution, on being exposed to reduced pressure,
gives up the gas, which is thus freed from the nitrogen of
the atmosphere. The sulphur dioxide is then mixed with
air and passed over the platinised asbestos heated to a
definite high temperature. Combination ensues, and the
sulphur trioxide is condensed in cooled receivers. It is a
white, crystalline, fuming substance, dissolving in water
with a hissing noise and with great evolution of heat. It
also unites directly with hydrogen chloride, with formation
of chloro-sulphonic acid, Cl— SO0-OH, a fuming very
corrosive liquid.
Sulphuric Acid. — The product on dissolving sulphur
trioxide in water is sulphuric acid, H2SO4 ; if smaller
156 MODERN CHEMISTRY
quantities of water be used than are necessary for the
formation of H.2SO4, various pyro- or anhydro-sulphuric
acids are produced, the simplest of which is the acid,
HO— (SO9)— O— (SO2)OH, analogous to some extent in for-
mula to pyrophosphoric acid, ( OH ) 2=PO-O-PO= ( OH ) 2,
but more closely resembling potassium dichromate. It,
too, is a fuming liquid, evolving much heat on addition of
water.
Sulphuric acid, however, is ordinarily made by bringing
together sulphur dioxide in presence of steam with nitric
peroxide, NO2, and oxygen. For this purpose, sulphur
or iron pyrites is burned in air ; the products of combus-
tion are passed through a flue provided with a chamber in
which it is possible to place, when required, a pot containing
a mixture of sodium nitrate and sulphuric acid ; the product
is nitric acid, which is at once attacked by the sulphur
dioxide, yielding sulphuric acid and nitric peroxide, thus :
2HNO3 + SO2 = H2SO4+ 2NO2. The gases next pass
up a tower, termed the " Glover tower," after its inventor.
In this tower they meet a spray of dilute sulphuric acid, the
decomposition product with water of a compound which
will afterwards be alluded to, hydrogen nitrosyl sulphate.
The hot gases, in contact with the dilute acid, evaporate
much of its water, which as steam finds its way along with
them up the tower. From the Glover tower the gases
enter the first of a series of leaden chambers, in which a
reaction occurs between the sulphur dioxide, the nitric
peroxide, and the steam, thus : SO2 + NO2 + H2O = H2SO4
+ NO. Excess of air is admitted along with the sulphur
dioxide, so that there is present in the leaden chamber a
considerable excess of oxygen. By its aid, the nitric oxide
is re-oxidised to peroxide, which is again attacked by the
sulphur dioxide, so that the nitric oxide serves as a carrier
of oxygen to the sulphur dioxide. The nitrogen of the air
conveys the gases from chamber to chamber ; and when it
has passed through a sufficient series (from nine to thirteen)
of chambers, all the sulphur dioxide has been converted
SULPHURIC ACID 157
into sulphuric acid, and deposited on the floors of the
chambers, whence it is run off from time to time ; it is
called " chamber-acid." Formerly, the nitric oxide and
peroxide used to escape into the air and be lost, besides
causing a nuisance ; to save it, G-ay-Lussac devised a
means of trapping it by passing the escaping gases up a
tower which bears his name ; a stream of concentrated
sulphuric acid flows down this tower, moistening the coke
or flint with which it is filled. On coming into contact
with the mixture of nitric oxide and peroxide, a salt of
sulphuric acid is formed — hydrogen nitrosyl sulphate,
HO— SO2— O— N=O, the group — N=O having replaced
one of the hydrogen atoms of the sulphuric acid. This
compound dissolves in the excess of sulphuric acid ; it is
conveyed by means of a special pump to the Glover tower,
where it is mixed with water, and is decomposed, thus :
2HO-SO2-O-NO + H2O = 2HO-SO2-OH = NO +
NO9. Although this compound is formed by the action
of concentrated sulphuric acid on a mixture of NO and
NO0, yet excess of water causes the action to proceed in
the opposite sense ; this affords a good example of the
action of mass.
After the chamber acid has been evaporated in leaden
vessels until a portion of the water is expelled, it is further
concentrated in vessels of platinum, glass, or iron. The
dilute acid is without action on lead, and the concentrated
acid does not attack platinum or iron, although iron is at
once dissolved by dilute acid. The heavy oily liquid
remaining after evaporation still goes by its old name of
" oil of vitrol." Its composition is not quite expressed by
the formula H2SO4, however, for that substance is unstable,
and parts with a trace of sulphuric anhydride when heated,
leaving a trace of water in the oil of vitrol. It can be
made by dissolving the right amount of anhydride in the
acid to combine with that water ; the resulting acid melts
at 10.5°; oil of vitrol has a much lower melting-point.
The molecular weight of liquid sulphuric acid, determined
158 MODERN CHEMISTRY
by its rise in a capillary tube, is very high, and appears to
correspond to about 3oH9SO4; on dilution it is no doubt
considerably lowered, and in dilute solution it is mostly in
the state of ions.
When heated to about 250°, sulphuric acid, as oil of
vitrol is usually termed, begins to emit fumes of anhydride ;
apparent ebullition takes place at about 350°, and the acid
distils over. This is, however, really dissociation into
anhydride and water ; for the density of the vapour is not, as
might be expected, half the molecular weight, 98, but only
24.5, one quarter of that number. And this agrees with
the theoretical density of a mixture in equal proportions of
the vapours of the anhydride and water, for (40 + 9) / 2 =
24.5. A considerable rise of temperature takes place on
mixing sulphuric acid with water ; it is not improbable that
the first anhydride of the true ortho-acid is formed ; the
compound of the formula H2SO4.H9O, which may be
O=S=(OH)4, melts at 8°. The point of maximum
contraction of a mixture of sulphuric acid and water occurs
when the proportion corresponds to H9SO4.9H2O ; this is
possibly S(OH)(J, but it does not easily solidify. Water
can be withdrawn from sulphuric acid by distilling it with
phosphorus pentoxide, when sulphuric anhydride is formed
and distils over.
Oxidising Action of Sulphuric Acid. — Sulphuric
acid can behave as an oxidising agent, being itself reduced.
This change is produced when it is heated with most other
elements. Thus with carbon, C + 2H9SO4 = CO2 + 2 SO.,
+ H,O; with sulphur, S + H2SO4 = 3SO2 + H?O; with
copper, mercury, iron, lead, silver, &c., a sulphate is formed,
and sulphur dioxide is liberated ; this may be viewed as the
reducing action of hydrogen, at the high temperature re-
quired for the reaction, thus : Cu + H2SO4 = CuSC>4 + 2H
and H2SO4 + 2H = 2H2O + SO2. The reduction goes
further, and some sulphur is liberated, while copper sulphide
is formed: CuSO4 + 8H = CuS + 4H,O ; H2SO4
SELENIC AND TELLURIC ACIDS 159
Hydriodic, and to a less extent hydrobromic acid also,
are oxidised by sulphuric acid: H9SO4 + 2HI = I0 +
2H0O + SO2 ; and alcohol and many other compounds of
carbon have a reducing action on sulphuric acid.
Selenic Acid. — Selenic Acid, H2SeO4, is also a
colourless syrupy liquid ; it can be produced by direct
oxidation of selenium by chlorine water, but on concentra-
tion the resulting hydrochloric acid reduces the selenic acid
to selenious acid, as hydriodic acid reduces sulphuric acid.
It is best prepared by addition of copper carbonate to the
mixture of selenic and hydrochloric acids obtained in that
way ; selenate and chloride of copper are formed ; the
mixture is evaporated to dryness, and the copper chloride is
dissolved out with alcohol, leaving the insoluble selenate
behind. The selenate is dissolved in water, and on treat-
ment with sulphuretted hydrogen, copper sulphide is pre-
cipitated, and removed by filtration ; the selenic acid is
then concentrated ; if it contains a trace of water, it is a
heavy liquid ; but if quite anhydrous, it forms a solid,
melting at 58°.
Telluric Acid. — Telluric acid is prepared from its
barium salt suspended in water, with the requisite amount
of sulphuric acid ; the barium salt is produced by heating
tellurium with barium nitrate. On evaporation, the hydrate,
H.)TeO4.2H0O, deposits in white crystals.
Sulphate, Selenates, and Tellu rates. — The sul-
phates, selenates, and tellurates of barium are nearly
insoluble in water ; those of strontium and lead are very
sparingly soluble, and those of calcium are still sparingly
soluble, though more easily soluble than the salts previously
mentioned. All these salts, therefore, are most conve-
niently prepared by the addition of a soluble sulphate,
selenate, or tellurate to a soluble salt of calcium, stron-
tium, barium, or lead, thus : CaCL. Aq + NaJSO4. Aq =
CaS04.2H20 + 2NaCl.Aq ; Pb(NOs)2.Aq + K.2SO4Aq
= PbSO4 + 2KNO3.Aq. These salts are still less soluble
in alcohol than in water, hence addition of alcohol to their
160 MODERN CHEMISTRY
solutions produces a turbidity. While barium sulphate is
not attacked by boiling hydrochloric acid, barium selen-
ate evolves chlorine, and is changed to barium chloride
and selenious acid, thus : BaSeO4 + 4HC1. Aq = BaCl0. Aq
+ H2SeO8. Aq + C12 + H2O.
All other sulphates are soluble in water, and can there-
fore be prepared by one of the usual methods, such as
treatment of the oxide, carbonate, or metal with the acid.
Dilute sulphuric acid dissolves magnesium, zinc, cadmium,
aluminium, chromium, iron, manganese, nickel, and
cobalt ; other metals resist its attack, because their electro-
affinity is less than that of hydrogen. The order is : Cs,
Rb, K, Na, Li, Ba, Sr, Ca, Mg, Al, Mn, Zn, Cd, Cr,
Fe, Co, Ni, Pb :— H : Cu, Hg, Ag, Ft &c., Au. All
the metals to the left of hydrogen in the table are attacked,
because they receive their ionic charge from the hydrogen
of the dilute acid: Zn + H?SO4.Aq = ZnSO4.Aq + H2 ;
the zinc is ionised, receiving its charge from the hydrogen,
which escapes in the molecular condition. But this trans-
ference of charge appears to require the contact of some
metal with lower electro-affinity than that of hydrogen, for
pure zinc is not attacked by pure dilute acid ; in fact, the
arrangement must be analogous to that of a battery. It is
possible that this is due to the protection of the zinc by a
film of condensed hydrogen — in other words, to polarisa-
tion ; contact with another metal affords a means of escape
of the charge from the hydrogen, which is evolved, not
from the surface of the zinc, but from the surface of the
less electro-positive metal.
With concentrated acid, these metals, as before remarked,
are dissolved as sulphates, with evolution of sulphur dioxide.
The sulphates form an important group of salts. Among
the best known are: Sodium sulphate, Na2SO4.ioH2O,
" Glauber's salt," contained in sea-water and in many
mineral springs; K2SO4, and (NH4).,SO4, hard rhombic
prisms ; the double salts, NaHSO4, and KHSO4, obtained
THE SULPHATES 161
by mixture ; when heated, these salts lose water and are
converted into pyrosulphates : 2 KHSO4 = H9O -f- K2S9O7.
CaSO4 occurs native, as anhydrite, and CaSO4.2H2(3, as
gypsum and alabaster. Gypsum, when gently heated,
loses its water, and is then known as " plaster of Paris ; "
on mixing it to a paste with water, combination takes place
slowly, and the plaster " sets ; " and in this way casts may
be taken. SrSO4 is found native as celestine ; BaSO4, as
heavy-spar or barytes. It is the commonest mineral
containing barium ; from it barium salts are prepared, by
heating it with ground coke, which reduces it to the sul-
phide : BaSO4 + 40 = ^.CO + BaS. The barium sulphide
is then dissolved in the appropriate acid, and the required
salt is made. Precipitated barium sulphate is known as
" permanent white ; " owing to its low price, it is much
used as a paint, although its covering power is small.
MgSO4.7H2O, ZnSO4.7H2O, and CdSO4.yH2O, as
well as FeSO4.7H2O, MnSOjyl-^O, and the correspond-
ing cobalt and nickel salts, are " isomorphous," that is, they
crystallise in the same form — rhombic prisms. Magnesium
sulphate, or " Epsom salts/' is present in sea-water and
in many mineral waters ; it also occurs in the salt deposits
at Stassfurth, in S. Germany, and is termed kieserite. It
is used as a purgative. Zinc sulphate is known as " white
vitriol," and ferrous sulphate as "green vitriol" or
"copperas." A large number of double salts exists, of the
formulas of which MgSO4.K0SO4.6H2O may serve as a
type ; they are all soluble, and they are ionised in solution
into the same ions as the simple salts would furnish ; thus,
+ + +
the ions of the salt mentioned above are Mg, 2K, and 2SO4.
They differ in this respect from such salts as K2SiF6, of
+
which the ions are 2K and SiF6. The alums form a
similar series of double salts, in which monad metals, such
as sodium, potassium, and ammonium, and triad metals,
aluminium, chromium, iron, manganese and others, are
combined together as sulphates with water of crystallisa-
VOL. II. L
i62 MODERN CHEMISTRY
tion: K2SO4.Al2(SO4)3.24H,O,orKAl(SO4),.i2H.2O.
The molecular weight of the compound is unknown ; hence,
as usual, the simpler formula is preferable. These com-
pounds are named from their analogy with the original
" alum," of which the formula is given above ; they all
crystallise in regular octahedra, and, like all true isomor-
phous salts, they are able to crystallise together ; so that if
a crystal of KA1(SO4).,. I2H.,O is placed, for example,
in a solution of (NH4)2Cr(SO4)2.i2H2O, the latter will
form a dark-red layer on the surface of the former.
" Alum " finds use as a mordant; when textile fabrics
are boiled in its solution, the fibre becomes incrusted with
a layer of aluminium hydroxide, and when subsequently
dyed the colouring matter is retained in combination with
the alumina and with the fibre, so that it cannot be removed
by washing. This phenomenon depends on the fact that
aluminium sulphate is partially hydrolysed by water into
A1(OH)3 and 3H2SO4; the adhesion of the alumina to
the fibre is attributed to " adsorption," a term applied to
the adhesion of gases, liquids, or of substances in solution,
to the surface of solids. A solution of alum also gives
coloured precipitates with many dye-stuffs, which are
known as " lakes." Selenic acid also yields alums.
Bismuth sulphate, Bi2(SO4)3, obtained by evaporating
a solution of bismuth oxide, Bi2Og, in sulphuric acid, forms
acicular crystals, which, on addition of water, like all other-
bismuth salts, yield a basic salt, in which the group O=Bi-,
bismuthyl, plays the part of a monad metal ; hence the
formula of the basic sulphate is (O=Bi)9SO4 ; it is an
insoluble powder. Copper sulphate, or " blue vitriol,"
CuSO4.5H2O, forms blue soluble crystals; silver, mer-
curous, and mercuric sulphates, Ag2SO4, Hg2SO4, and
HgSO4, are sparingly soluble, white crystalline powders.
As the ion SO4 is colourless, all these salts possess the
-¥ +
colour of the metallic ion which they contain ; thus, Fe
SALTS OF RADICALS 163
+ 4- + + + + + + ++ + +
is green, Mn pink, Cr green, Fe yellow, Ni green, Co
red, Cu blue, and the others colourless.
Sulphates of the alkali- and alkaline-earth metals are
stable at all temperatures lower than that of the electric arc ;
but all other sulphates decompose, the primary product being
the oxide of the metal and sulphuric anhydride ; the latter,
however, being unstable at a red-heat, decomposes partly
into sulphur dioxide and free oxygen. This decomposition
is made use of in the preparation of " Nordhausen sulphu-
ric acid," a fuming liquid, consisting chiefly of H9S9O7 ;
it is made by distilling partially dried ferrous sulphate from
fireclay retorts: 2FeSO4= Fe9O3 + SO2 + SO3 ; the
pyrosulphuric acid is produced by the union of the anhy-
dride with water: 2SO3 -f- H2O = H9 S9Or The iron
oxide has a fine red colour, and is sold as a paint under the
name " Venetian red."
Salts of Alkyl Radicals. — Salts of the alkyl radi-
cals are as a rule volatile ; they are produced by distilling
the alcohols with the respective acid. Ethyl nitrite, for
example, is formed by distilling a mixture of alcohol,
sodium nitrite, and sulphuric acid : NaNO9 + H9SO4. Aq -h
C9HrOH = C,HrNO0 + NaHSO4.Aq. " It is a volatile
2 5 - 5 -• 4 T.
liquid, with a pleasant odour, which, when boiled with
potash, is hydrolysed, with formation of sodium nitrite and
ethyl alcohol : C9H5NO2 + KOH.Aq = K-O-N=O.Aq
+ C2H5OH. The nitrate, C,H5ONO2, cannot be pre-
pared from nitric acid and alcohol unless the presence
of nitric peroxide is excluded ; for this purpose urea,
CO(NH9)2, is added in small proportion to the mixture ; its
presence prevents the oxidation of the alcohol, and brings
about the normal action C2H5OH + HNO3 = C2H&NO3 -f-
H9O. The nitrate resembles the nitrite in properties. On
mixing alcohol with sulphuric acid there is a considerable rise
in temperature, and hydrogen ethyl sulphate is produced :
C2H5O H + HO-SO2-OH = HO-SO2-OC 9H5 + H2O.
A considerable excess of sulphuric acid must be present in
164 MODERN CHEMISTRY
order to ensure the nearly complete conversion of the
alcohol into the ethyl salt. To remove this excess, calcium
carbonate is added, which forms sulphate of calcium and
a double sulphate of ethyl and calcium, Ca(C9H5SO4)2 ;
the former is nearly insoluble in water, while the latter is
readily soluble ; from the calcium salt the acid may be pro-
duced by addition of the theoretical amount of sulphuric
acid. On evaporation it is a syrupy liquid ; it decomposes
when heated into ethylene, sulphur dioxide, carbon mon-
oxide, and carbon dioxide. As seen by the formula of the
calcium salt, the acid is a monobasic one. The potassium
salt, for example, has the formula K(C^H5)SO4 ; the salts
are all soluble. Similar acids are formed from other alkyl
radicals, such as methyl, amyl, &c.
ThlOSltlphates. — Some other acids of sulphur remain
to be noticed. Among these is thiosulplmric acid,
H2S2O3, of which the sodium salt is produced by digesting
together sodium sulphite with sulphur, just as, with oxygen,
sodium sulphate is formed. In the latter case it may be
supposed that the atom of oxygen inserts itself between the
sodium atom and the sulphur atom with which it is in com-
o o
li i:
bination, thus : Na-Q-S-Na + O = Na-Q-S-Q-Na ;
I! li
O O
o o
I. ii
Na-O-S-Na + S = Na-O-S-S-Na ; hence the name
I I1
O O
" thio " sulphate, the " thion," or sulphur, replacing the
oxygen of sulphuric acid. The sodium salt forms large
transparent crystals of the formula Na2S2O3.5HQO ; the
barium salt is sparingly soluble, and forms a crystalline
precipitate on adding a solution of the sodium salt to one of
barium chloride ; the lead salt is insoluble, and the silver
salt is a white precipitate, which rapidly turns dark on
IODOMETRY 165
application of heat, being converted into silver sulphide :
Ag2S2O3 + H2O.Aq-Ag,S + H2SO4.Aq. On acidify-
ing any one of the soluble salts, the acid is momentarily
liberated ; but it immediately decomposes into sulphurous
acid and sulphur, H9S2O3.Aq = H0SOg.Aq + S, the latter
rendering the liquid milky. The sodium salt, when a solu-
tion of iodine in one of potassium iodide is added to it,
undergoes the reaction : 2Na0S.7O3. Aq + I0. Aq = zNal. Aq
+ Na9S4O6. Aq. The salt formed is named tetrathionate
of sodium. It will be considered shortly.
lodometry. — A solution of sodium thiosulphate con-
taining 248 grams, made up to a litre with water, reacts
quantitatively with one containing 127 grams of iodine per
litre ; the colour of the iodine disappears, and the vanishing
of the last trace of iodine can be ascertained by the addition
of some starch paste, which gives a blue colour so long as
any free iodine remains unconverted into ions ; such a solu-
tion is commonly used in estimating iodine, or in determin-
ing the quantity present in solution of any substance which
has the property of liberating iodine from acidified iodide,
i.e. from hydriodic acid, such as free chlorine, a hypo-
chlorite, or, indeed, any oxidising agent.
On boiling together solutions of sodium thiosulphate with
ethyl iodide, sodium ethyl thiosulphate is formed ; its
O
ii
formula is Na~ O~ S~ S~C9H5, for, when mixed with
6
barium chloride, the barium salt, which is unstable, decom-
poses into barium dithionate and ethyl disulphide, thus :
X)-SO.-S(C0H5) /O-SO0 S(C,H.)
Ba< = B< I ^ + I
M>-SO2-S(C2H5) X)-SO2 S(C2H5)
This decomposition renders two suppositions probable :
CK SH
that thio-sulphuric acid has the formula /S\ »
\OH
166 MODERN CHEMISTRY
,
and not ^^\ > ar>d tnat dithionic acid is constitu-
O^ \OH
O2S-OH
tionally represented by I
O2S-OH
Hydrosulphites. — Hydrosulphurous acid, H2S2O4,
sometimes called " hyposulphurous acid," is produced
as zinc salt by the action of metallic zinc on sulphurous
acid. The liquid turns brown, and possesses great reducing
power. The sodium salt, which is better known, is pro-
duced by digesting zinc turnings with a concentrated solu-
tion of hydrogen sodium sulphite : Zn + 4HNaSO3. Aq =
Na2Zn(SO3)2 + Na2S2O4.Aq + 2H2O ; the sodium zinc
sulphite crystallises out on addition of alcohol, leaving the
hydrosulphite in solution. On cooling the solution, slen-
der crystals of the hydrosulphite separate. The solution
absorbs free oxygen so rapidly that it turns warm ; it is
used as a reducing agent in indigo-dyeing ; indigo-blue is
converted by the hydrogen of water (of which the oxygen
enters into combination with the hydrosulphite, convert-
ing it into sulphite) into a colourless substance, termed
indigo-white ; this body being soluble, penetrates the fibre
of fabrics dipped into the solution, and on exposure to air,
indigo-blue, with its usual colour, is deposited as an in-
soluble precipitate in the cloth. By help of sodium hydro-
sulphite, too, a ferrous salt may be deprived of ferric so
completely that it gives a nearly white precipitate with
alkalies ; the usual colour of the impure ferrous hydroxide
is a dirty green.
Thionates* — Manganese dithionate is produced by
passing a current of sulphur dioxide through freshly pre-
pared manganese dioxide suspended in water, made by
boiling potassium permanganate with alcohol. The equa-
tion MnO2.nH2O + 2SO2.Aq = MnS2O6.Aq represents the
change. On addition of barium hydroxide to the man-
ganese salt, manganous hydroxide is thrown down, and
THIONATES 167
barium dithionate is left in solution. From it, the acid
may be prepared by the addition of the requisite amount of
sulphuric acid ; and the other salts, by addition of the
appropriate sulphate. The acid, concentrated by evapora-
tion at a low temperature, is a sour, syrupy liquid ; when
heated, it decomposes into sulphur dioxide and sulphuric
acid.
Trithionic acid, H2S3O6, is also known ; it is a still
more unstable liquid.
Tetrathionate of sodium, as already remarked, is pro-
duced by addition of a solution of iodine to a thiosulphate ;
it is precipitated on addition of alcohol. The acid forms
a colourless solution, with strong acid taste. The
method of its formation gives a clue to its constitution :
NaO-S(O9)-S-Na I Nal NaO-S(O2)-S
+ I = + I •
NaO-S(02)-S-Na I Nal NaO-S(O2)-S
Pentathionic acid, H9S5O6, is produced by passing a
current of hydrogen sulphide through a dilute solution of sul-
phurous acid, along with tri- and tetrathionic acids : 5Ht>8
+ 5SO2.Aq. = H2S5O6.Aq + 4H2O + 58 is the equation
usually given. Excess of hydrogen sulphide, passed for a
long time, results in the reaction 2H9S + SO9.Aq =
2H2O.Aq + 3S. The tri-, tetra-, and pentathionates,
when heated, yield a sulphate, sulphur dioxide, and free
sulphur.
Highly Oxidised Acids- — Of recent years, a con-
siderable number of salts of acids more highly oxidised
than any of those already mentioned has been prepared. It
has long been known that on addition of hydrogen dioxide
to a solution of potassium bichromate acidified with
sulphuric acid, a bright blue colour is produced, and that
this coloured substance can be extracted by ether from its
aqueous solution. The compound has recently been
identified as perchromic acid, CrO4(OH); for, on
adding to the cooled blue solution a solution of ammonia
in ether, a violet precipitate of CrO4(O— NH4).H0O0 is
168 MODERN CHEMISTRY
thrown down ; and if an etherial solution of potassium
hydroxide be added, the potassium salt of similar formula
is precipitated. These bodies are explosive.
Persulphates of potassium and ammonium are produced
by passing a current of electricity through concentrated
solutions of the sulphates in water. The persulphate is
sparingly soluble, and deposits in white crystals. The
formula appears to be M,,S0O8 (M = monad metal).
The acid has bleaching powers, and gradually decomposes
into sulphuric acid and ozone.
Perborate of sodium, NaBO3,4H0O, is similarly pre-
pared, or it may be produced by cooling a solution of
borax to which some caustic soda and hydrogen peroxide
have been added. It, too, is a sparingly soluble salt, pos-
sessing bleaching properties.
Percarbonate of sodium, Na0CO4. iiH0O, is similarly
prepared by addition of alcohol to a solution of sodium
carbonate, to which a solution of hydrogen peroxide has
been added. It is a white, extremely unstable compound,
possessing, like the other similar salts, great oxidising
power.
CHAPTER IX
The Nitrides and Phosphides, Arsenides and
Antimonides — Complex Amines and their
Salts — Acid Amides — The Cyanides and the
Double Cyanides.
Analogy between Oxides and Nitrides. — Nitro-
gen and phosphorus are best characterised by the compounds
in which they act as triads. For just as an oxide or
hydroxide may, as was customary during the era of the
theory of " types," be regarded as water in which the
atoms of hydrogen are more or less completely replaced by
atoms of a metal ; so from analogy it is to be inferred that
compounds should be preparable which should be similarly
related to ammonia and to hydrogen phosphide. The
following graphic formulae will render the conception
clear : —
H-O-H -> Na-O-H -+ Na-O-Na ;
H-NH2 -> Na-NH2 -> Na-N=Na2 ;
H-O-H ) 0-H
>
> - Ca
H-O-H ) 0-H
H-NH2 ) ,NH2 ,
H-NH0 ) \NH2
169
170 MODERN CHEMISTRY
H-O-H) /OH XO
H-O-H I -> Cr^ OH -> Cr^ ->
H-O-H XOH \OH Cr/
H-NH0 ) /NH0 /NH
H-NH: y -> c< NH; -> c^ -> CI=N.
H-NH" ) NtfHg \NH2
But although most elements combine with oxygen directly,
there are only a few which burn in nitrogen. Among these
are lithium, calcium, and magnesium ; boron and titanium
also possess this property. The nitrides of the other
elements are practically unknown. These nitrides are
attacked by water ; the three first with violence at the
ordinary temperature ; the two last, when heated in a
current of steam. The products are the hydroxide of the
metal and ammonia ; or with boron and titanium, owing to
the high temperature of action, the oxide, thus : Mg3N2
+ 3H2O.Aq= 3Mg(OH)2 + 2NHrAq.
Nitrides. — Lithium nitride, LigN, is a dark-coloured
substance ; it is formed at the ordinary temperature on expos-
ing metallic lithium to the air. Calcium nitride, Ca.N9, is a
greyish-yellow substance ; and magnesium nitride, Mg3N2,
a yellow powder. Combination takes place readily with
great evolution of heat when a mixture of dry lime with
magnesium powder is heated to dull redness in a current of
nitrogen ; this affords a convenient method of separating
nitrogen from the indifferent gases of the atmosphere, and
preparing the latter in a state of purity. Boron nitride,
BN, is a white amorphous powder ; it can also be produced
by heating to redness a mixture of boron oxide with ammo-
nium chloride, until excess of the chloride has volatilised.
Hydrazoates. — Besides these compounds, which may
be regarded as the analogues of the oxides, a series of
nitrides is known, which correspond in formula with
hydrazoic acid, HN3. The starting-point for these com-
pounds is sodamine, NaNH2 (see below). This compound
HYDRAZOATES 171
is heated to 300° in a series of small flasks in a current of
nitrous oxide, when the following reaction takes place :
2NaNH2 + N2O = NaN3 + NaOH + NH3. The change
which takes place is more obvious when the reaction is con-
/N /N
ceived to occur in two stages: NaNH0 + O< II = NaN/
\N \N
+ H2O; and NaNH, + H2O = NaOH + NH3. The
product of the reaction is dissolved in water, acidified with
dilute sulphuric acid, and distilled : NaNg.Aq + H0SO4. Aq
= HN3.Aq + NaHSO4. Aq. The distillate, which is a
dilute solution of hydrazoic acid, has a peculiar odour, and
if its vapour be inhaled, fainting may result ; it is necessary
to take precautions to distil it in a good draught. The
solution has an acid reaction ; salts may be prepared by
neutralisation with the hydroxides or carbonates of the
metals. The ions, -N3, are colourless, and the salts of
colourless ions are themselves white. Those of lithium,
sodium, potassium, magnesium, calcium, strontium,
barium, and zinc are crystalline ; their formulae are M'N3
and M//(N3)2 respectively. Silver hydrazoate, AgN3,
closely resembles the chloride in appearance and in in-
solubility ; it is, however, dangerously easy to explode,
and should be prepared dry only in minute quantity, and
treated with the utmost precaution. Titration with a
deci-normal solution of silver nitrate affords a convenient
method of determining the strength of a solution of
hydrazoic acid, or of analysing the hydrazoates ; it is easy
to recognise the point when all hydrazoic acid has been
removed as the insoluble silver salt.
Amines. — Substituted ammonia, in which one atom of
hydrogen is replaced by an element, is the analogue of the
hydroxides. Such bodies are termed amines or amides.
Sodamine, NaNH2, is easily prepared by passing a current
of ammonia, dried by passing it through a tower filled with
soda-lime, through an iron U-tube containing sodium, and
heated to about 300°. The gas is rapidly absorbed, while
172 MODERN CHEMISTRY
hydrogen is evolved : 2NH3 + 2Na = 2NaNH0 + H2.
When the sodium has been all converted into sodamine,
the tube is emptied by pouring out its contents. Sodamine
is a white brittle substance, with crystalline fracture, not
unlike caustic soda, melting at about 100°. So long as it is
kept dry it is quite permanent, but with moisture it at once
reacts, forming ammonia and caustic soda : NaNH.., + HOH
= NaOH + NH3. Similar compounds can be made with
lithium, potassium, rubidium, and probably caesium.
The corresponding compound of zinc, Zn(NH0)9, is a
white powder, insoluble in ether, formed along with ethane
or methane by the action of ammonia on zinc methide or
ethide : Zn(CH8)2 + 2NH3 = Zn(NH2)2 + 2CH4.
Guanidine. — An attempt to produce the amine of
carbon, C(NH2)4, by the action of ammonia on such
a body as carbon tetrachloride or ethyl orthoformate,
C(OC9Hr)4, according to the equations CCl4 + 4NHg
= C(NH2j +4HC1, or C(OC2H5)4 + 4NH3 = C(NH2)4
+ 4HOC9H5, fails, owing to loss of ammonia. For just
as orthocarbonic acid, C(OH)4, loses water, yielding
ordinary carbonic acid, so carbon tetramine loses ammonia ;
the product is named guanidine, and has the formula
HN = C(NH2)2; its analogy with O = C(OH)2is easily
seen. Guanidine is a white crystalline substance, which,
like ammonia, unites with acids to form salts.
On comparing the formulas of carbonic acid and guani-
dine, it is evident that several intermediate compounds
should be capable of existence. The series is : —
NH2
0 = C(OH)0 HN = C(OH)0 0 = C<
X)H
(0 (2) (3)
/OH
O = C(NH9)2 HN = C< HN = C(NH0).7.
XNH0
(4) 15) (6)
CARBAMATES 173
Of these, the best known are the ammonium salt of (3),
which is termed carbamic acid, and (4), the important
compound urea or carbamide.
Carbamates. — Ammonium carbamate, known by
the familiar name of " smelling salts/' is formed by mixing
ammonia and carbon dioxide gases : CO9 + 2NHg =
H2N - CO - ONH4. It is a white crystalline compound,
soluble in water and smelling of ammonia. Its solution,
when fresh, contains the compound of which the formula
is given above ; but after standing, it is converted by
absorption of water into ammonium carbonate. This has
been ascertained by treating the freshly prepared solution
with sodium hypochlorite, when only half the nitrogen
which the substance contains is evolved :
2H0N-CO-ONH4. Aq + sNaOCl. Aq =
2H2N-CO-OH.Aq + 3H20 + 3NaCl + N2 ;
on the other hand, with a hypobromite, all the nitrogen
is evolved :
H0N-CO-ONH4.Aq + 3NaOBr. Aq = CO2 +
Now, ammonium salts yield up their nitrogen when
mixed with a solution of a hypochlorite ; hence it is con-
cluded that the compound contained in a fresh solution
is ammonium carbamate. But on standing, the solution
changes, and after some time it yields all its nitrogen
on treatment with hypochlorite ; hence the assumption of
the elements of water and a change into ammonium car-
bonate may be inferred: H2N-CO-ONH4.Aq + H,O -
H4N— O— CO— ONH4. Aq. But ammonium carbamate may
conceivably possess the formula HO— C(NH)— ONH4 ;
and it may be that it is the =NH group which resists
attack. This last supposition is confirmed by the behaviour
of urea with hypochlorite ; for with it, too, only half the
nitrogen is evolved.
Carbamide. — Urea or carbamide, to which the for-
174 MODERN CHEMISTRY
mula O = C(NH>>)>2 is generally ascribed, is the form in
which by far the largest part of the nitrogen is evolved
which is consumed as food by animals. It may be directly
prepared from urine by evaporation to one-third of its bulk,
and addition of nitric or of oxalic acid ; the sparingly
soluble nitrate or oxalate is precipitated; the salt is purified
by recrystallisation from water, and is then mixed with
caustic soda and evaporated to dryness. On treatment
with alcohol, the urea alone dissolves, and deposits in
crystals from a concentrated solution. It is a white, easily
soluble substance, with a saline taste. It unites with acids,
forming salts ; but as the carbonyl group, CO, has the
property of conferring acidity on neighbouring atoms of
hydrogen, the basic qualities of only one of the two
amido-groups, — NH2, can display itself; hence the for-
mula of the hydrochloride is CO(NH2)2.HC1, and not
CO(NH2)2.2HC1, as might be expected. It is therefore
a mono-acid base.
Urea can also be produced from inorganic sources, and
it was the discovery of its synthesis from potassium cyanide
by Wohler in 1827 which caused the abandonment of the
old view that compounds containing carbon, with the ex-
ception of its oxides, belonged to a special class, and could
be produced only by the intervention of "life-power." Its
production is as follows : Potassium cyanide is heated to
redness with lead oxide; KCN + PbO = KCNO + Pb.
The cyanate, KCNO, is next dissolved and mixed with a
solution of ammonium sulphate, and the mixture is evapo-
rated to dryness. It may be supposed that potassium
sulphate and ammonium cyanate are first formed: 2 KCNO
+ (NH4)2SO4 = K2SO4 + (NHJCNO. But the latter
compound is unstable, and undergoes change into its
isomeride, urea : (NHJCNO = O=C(NH2)2. On treat-
ment with alcohol, insoluble potassium sulphate remains
undissolved, while the soluble urea crystallises from the
alcohol on evaporation. Urea is also produced when carbonyl
chloride or when ethyl carbonate is treated with aqueous
AMIDES OF PHOSPHORUS 175
ammonia : O=CC10 + 2NH3 = O=C(NH2)0.HC1 +
HC1 ; 0=C(OC?H5)? + 2NH3 - Q-C(NH2)2. +
2C9H5OH. Lastly, carbamate of ammonium, when heated
in a sealed tube, loses water with formation of urea :
H2N-CO-ONH4 - 0=C(NH2)2 + H20.
Bill ret. — When urea is heated, a body named biuret is
formed, with loss of one molecule of ammonia. We are here
reminded of the relation between an acid and an anhydro-
acid ; this is evident on inspection of the formulas : —
H2N-CO-NH2 HO-S02-OH
Urea. Sulphuric acid.
H2N-CO-NH-CO-NH2 HO-SO2-O-SO2-OH
Biuret. Anhydrosulphuric acid.
Amides of Acids of Phosphorus. — Many com-
pounds analogous to urea are known, where the hydroxyl
groups of acids are replaced by amide-groups, — NH2. By
the action of ammonia gas on phosphorus oxychloride
ortho-phosphamide is formed : O=PC13 + 3HNH2 =
O=P(NH2)3 + 3HC1. The ammonium chloride formed by
the combination of the hydrochloric acid with excess of
ammonia is removed by washing, and an insoluble white
powder remains. When phosphamide is heated, ammonia
is lost, and phosphoryl-amide-imide (the group =NH
is termed the " imido-group "), HN=PO(NH2), and at
a higher temperature, N=P=O or phosphoryl nitride are
left. They are also white insoluble powders. By analogy
with carbamic acid and urea, there should exist compounds
in which both hydroxyl and the amido-group are present.
Some such compounds are known. Thiophosphamic
acid, S=P(NH2)(OH).2, is the product of the action of
ammonia on thiophosphoryl chloride ; and phosphoric
anhydride, when dry ammonia gas is passed over it, yields
o.
phosphimic acid, thus : P9Or + zNIHL =
176 MODERN CHEMISTRY
+ H,O. It is analogous to metaphosphoric acid,
ov
^P — OH, and forms crystalline salts. Pyrophos-
o^
phamic acids are also known. The addition of phos-
phoryl chloride to a cold saturated solution of ammonia
results in the formation of pyrophospho- diamic acid
(PO) — O — (PO)<^ , analogous to pyrophos-
H,N/ X)H
HOX /OH
phoric acid, >(PO)— O— (PO)< ; and on
HO/ \OH
heating the solution of this body, one hydroxyl group
replaces one amido-group, yielding pyrophosphamic acid,
H0N, /OH
\(PO)— O— (PO)< . Lastly, the action of
HO/ X3H
ammonia on phosphoric chloride gives a compound named
phospham, HN^P^N, a species of anhydride, but pro-
duced by loss of ammonia, not of water, from the unknown
compound P(NH2)_.
Analogues of phosphorous acid are less well known ; if
ammonia be passed over phosphorous chloride, a white mass
is formed, which has not been separated from ammonium
chloride, but which is supposed to possess the formula
P(NH2)3; it may be named phosphorosamide.
.A/n/cfes of Sa/p/lt/r Ac/c/S. — Similarly, amido-
derivatives can be obtained from sulphur trioxide. The
action of ammonia on that compound yields ammonium
sulphamate, H4N— O— (SO2)— NH2, or, if less ammonia
be used, sulphamic acid, HO— (SO0)— NH2 ; they are
both crystalline, soluble compounds.
The action of sulphur dioxide on ammonia is accompanied
by the production of the analogous compounds, ammonium
sulphurosamate and sulphurosamic acid, the latter of
which has the formula RO-(SO)-NH2.
COMPLEX AMINES 177
These compounds may be taken as instances of bodies
analogous to acids, in which the hydroxyl is replaced by
the amido-group. They are, as a rule, stable in presence
of water, and they do not generally unite with acids, the
acid nature of the oxygen which they contain counteracting
the basic nature of the amido-group. Many compounds
are however known, in which the amido-group replaces
hydroxyl, and which, having no acidic oxygen present, are
known only as salts in combination with acids. Some of
these will now be described.
Salts of Complex Amines. — Calcium chloride,
exposed to a stream of ammonia gas, rapidly absorbs it,
and forms the compound CaCl2.8NH3. It would appear
that this compound is one of calcamine, Ca(NH9)9, with
2HC1, with which six molecules of ammonia are associated
in some manner resembling that in which water of crystal-
lisation is associated in salts containing it. Thus we have
CaCl9.6H9O ; and Ca(NH3)9Cl.2.6NH3 has an analogous
formula. Zinc and cadmium form similar compounds,
and other salts may be obtained from the appropriate salts
of the metals ; thus, by saturating zinc sulphate with
ammonia, the compound Zn(NH3)9SO4.H9O separates
in crystals. Again, with aluminium, A1(NH3)3C13 has
been prepared ; and dyad iron, manganese, and nickel
yield somewhat similar compounds. Such bodies must
be regarded as salts of ammonium, in which a metal
has taken the place of one atom of hydrogen in each
molecule of ammonium ; a dyad metal replacing two
metals in two molecules of ammonium, a triad three,
and so on.
The state of such compounds in solution is probably
that of "double salts," alluded to on pp. 10 and 161.
While some of them are decomposed by water into ammonia
and the salt of the metal, others resist that decomposition,
and are ionised into complex groups, analogous to the platini-
chloride or the silicifluoride group. Thus, while it is
probable that the compound of ammonia with calcium
178 MODERN CHEMISTRY
+ +
chloride in solution contains as ions Ca, Cl, NH4, and
OH, together with non-ionised NH4OH and molecular
NH3, the fact that zinc hydroxide, precipitated by addition
of ammonium hydroxide to a solution of the chloride, is
re-dissolved by further addition of ammonia, is doubtless
to be explained by the formation of the complex ion
Zn(NH3)2, which is soluble in water. But this does not
+ +
exclude the presence of the usual ions, Zn, Cl, NH4, and
OH, which doubtless co-exist with those already mentioned.
In some cases, the stability of the complex ions is much
greater than in that mentioned, and of this some instances
will be given.
Chromamines. — Chromium hydroxide,when digested
with excess of ammonia and ammonium chloride, forms a
deep red solution ; and on exposing it to air, a violet
powder separates, of the formula CrCl3.4NH3.H9O. This
powder, heated to 100°, loses its water of crystallisation,
and the residue has the formula CrCl3.4NH3. The
ammonia is not expelled until the temperature 200° is
reached. It would appear, therefore, that this compound
is not Cr(NH3)3.Cl3.NH3, the fourth molecule of ammonia
being regarded as of the same nature as water of crystallisa-
tion ; it must rather be supposed that a complex ammonium
group, — NH3— NH3, is capable of existence ; whence the
^
compound would have the formula Crc — NH0 — NH0 — Cl.
\NH'-CI '
Salts containing chromium have been prepared, in which
3, 4, 5, 6, and 7 molecules of ammonia are associated with
the original chromium salt. They find their explanation
by a hypothesis like the one given.
Cobaltamines. — Similar compounds are known with
triad cobalt. On adding a solution of ammonia to cobalt
sulphate, the precipitate at first formed (a basic sulphate)
AMINES 179
dissolves ; exposure to air causes the oxidation of the cpbalt
from dyad to triad, and a black powder is deposited. On
careful addition of hydrochloric acid, keeping the mixture
cold, the colour of the powder changes to red ; the com-
/NH3— Cl
pound has the formula Coc-NIHL — NH, — C1.H0O, and is
\NH3— Cl '
analogous to the chromium compound mentioned above.
Other salts of this base have been made ; they are termed
roseo-cobaltamines. If temperature be allowed to rise,
during the addition of hydrochloric acid to the oxidised
solution of cobalt sulphate in ammonia, an isomeric substance
is produced, containing no water of crystallisation, and
having a purple colour. Other salts are known ; they are
termed salts of purpureo-cobaltamine. It is possible to
represent the formulas of such compounds as follows : —
Diamines: Cl-Co(NH3)2Cl.,.
Triamines : Co (NH« ) £ly
Tetramines : Cl-Co(NH3-NH3)9Cl9.
Pentamines : NH9-Co(NH-3NH3).;Cl3.
Hexamines: Co(NH3-NH3)3Cl3.
Other Amines. — Many compounds of copper, mer-
cury, silver, gold, and the metals of the platinum group
are known, which admit of representation in a similar
manner. They differ, however, inasmuch as the metal
must be considered to have replaced more than one atom of
hydrogen in one molecule of ammonium. Thus we have :
Cu'2=NH2Cl, di-cuprosammonium chloride, a black
powder produced by the action of ammonia gas on warmed
cuprous chloride :
Cu'— NH3C1, cuprosammonium chloride, formed by
dissolving cuprous chloride in ammonia ; it is a well-known
absorbent for carbon monoxide and for acetylene.
Cu"=(NH3)9Cl9, cuprammoniuin chloride, and cupri-
diammonium sulphate, Cu"=(NH3— NH3)9SO4, the
former a green substance, the latter a deep blue compound ;
iSo MODERN CHEMISTRY
both produced by the action of ammonia on the respective
cupric salt. The formation of the latter is a well-known
test for copper.
With silver there are : Argentamine, a black ex-
plosive powder, probably of the formula AgNH«, produced
by adding ammonia to silver hydroxide ; and numerous
compounds of formulas like Ag(NH3)Cl, produced by
dissolving the respective silver salts in ammonia. With
gold : Auric chloride, digested with ammonia, yields
"fulminating gold," an explosive black substance, which is
a mixture of HN= AuCl and HN=Au-NH.,.
A familiar test for mercurous mercury is to add ammonia,
when the compound turns black. This is due to the
formation of di-mercurosammonium chloride, or some
similar compound, of the formula Hg'0=NH2Cl, where two
atoms of hydrogen in ammonium chloride are replaced by
twro atoms of monad mercury. It has long been known,
too, that mercuric salts produce a white precipitate on treat-
ment with ammonia. This is chiefly due to the formation of
chloro-mercuramine, Cl — Hg" — NH2 ; here, the presence
of electro-negative chlorine deprives the amido-group of
basic properties. On boiling this compound with ammo-
nium chloride, mercurammonium chloride is produced:
Cl-Hg-NH2 + NH4Cl = Hg(NH3)2Cl,.
With platinum, and the other members of that group,
similar compounds are produced ; but their constitution can
be inferred sufficiently from what has preceded.
These compounds are derivatives of ammonia ; there are
few similar compounds of phosphine ; one, however, is
produced when phosphoretted hydrogen is passed over
aluminium chloride; its formula is PH^AlClg. And
arsine, passed through a solution of mercuric chloride, yields
Hg9AsCl.HgCl2 ; it is somewhat analogous to the black
precipitate, Hg2NH.HCl.
Phosphides, Arsenides, and Antimonides. — A
few compounds of phosphorus, arsenic, and antimony with
metals have been made. They are generally obtained by
PHOSPHIDES, ETC. 181
direct union between the heated metal and the element.
Thus, if sodium and phosphorus be heated together under
xylene, a hydrocarbon boiling about 130°, a black com-
pound is formed, NagP, from which excess of phosphorus
can be dissolved out by treatment with carbon disulphide.
Arsenide and antimonide of sodium are also obtained by
heating the elements together. The formulas of these
compounds are of the type AsNa3 ; and with dilute acid,
the corresponding hydride of phosphorus, arsenic or antimony
is evolved : AsNa3 + 3HCl.Aq = AsH3 + 3NaCl.Aq.
A mixture of calcium phosphide, Ca3P2, with calcium
pyrophosphate is produced on throwing phosphorus into a
crucible containing red-hot lime ; on treatment with water,
spontaneously inflammable phosphine is evolved. The
spontaneous ignition is due to its containing P0H4, a liquid,
very unstable compound.
The phosphides, arsenides, and antimonides of the
other metals are usually dark-coloured substances, with
more or less metallic lustre, and therefore conductors of
electricity. Some of them occur native ; for example,
smaltine, CoAs2, a common ore of cobalt, forming silver-
white crystals ; copper-nickel, NiAs, red lustrous crystals,
and one of the chief nickel ores ; speiss, a deposit formed
in the pots in which smaltine and copper-nickel are fused
with potassium carbonate and silica, in the preparation of
smalt, a blue glass containing cobalt ; its formula appears to
be Ni3As2. Mispickel, or arsenical pyrites, is a white
lustrous substance, of the formula FeSAs.
Cyanides. — The elements carbon and nitrogen form a
very stable group, of which the compounds have been well
investigated, termed cyanogen. Carbon and nitrogen do
not unite directly ; but if a mixture of finely divided carbon
with carbonate of potassium or sodium, or, better, of barium,
be heated to about 1 200° in a current of nitrogen, combina-
tion ensues, and a cyanide is formed, KCN, NaCN, or
Ba(CN)2; BaC03 + 4C + N2 = Ba(CN)2 + 3CO. Potas-
sium cyanide is also produced when a mixture of animal
i82 MODERN CHEMISTRY
refuse (horns, hides, hair, dried entrails, &c., of animals)
with potassium carbonate and iron filings is heated. The
nitrogen of the animal matter and the carbon unite with the
potassium of the carbonate, forming cyanide. On addition
of water, this cyanide reacts with salts of iron, forming a
double cyanide of iron and potassium, termed " yellow
prussiate of potash," or ferrocyanide of potassium, of the
formula K4Fe(CN)6. When this compound is heated to
dull redness, it fuses ; a black mixture or compound of iron
and carbon remains, and melted potassium cyanide can be
poured out of the crucible. Potassium cyanide, KCN, is a
very soluble salt ; it crystallises well from alcohol. Its
solution smells of hydrocyanic acid ; this is because it is
hydrolysed by water. The acid, HCN, is so very weak
that the number of hydrogen ions present in its solution are
comparable in number with those of ionised water ; hence
the change: H-OH + K-CN.Aq=HCN + KOH.Aq.
The ionised portion of the hydrocyanic acid is as usual non-
volatile ; but the non-ionised portion has a vapour-pressure,
and can be detected by its smell (cf. p. 141).
Hydrocyanic Acid. — Inasmuch as hydrocyanic or
" prussic " acid is so very weak, it is displaced from its
salts by the action of all other acids ; even carbonic acid
can expel it. It may therefore be prepared by distilling a
cyanide (potassium cyanide or ferro-cyanide is generally
used) with dilute sulphuric acid. The vapour comes off
along with water ; to remove the water, if the anhydrous acid
is required, the mixture of vapours may be passed through
a tube charged with dry calcium chloride, which retains
the water ; or by another method, mercuric cyanide may be
decomposed by passing over it a current of dry sulphuretted
hydrogen, the excess of which is removed by causing the
gases to traverse a tube filled with lead carbonate ; the
hydrocyanic acid must be condensed in a freezing-mixture,
for it boils at 27°. The solid compound melts at -I 5°. All
experiments with anhydrous or with concentrated hydrocy-
ISOMERIC CYANIDES 183
anic acid must be carried out in a good draught-chamber,
for it is the most poisonous substance known, and a breath
of its vapour has been attended by fatal effects.
The cyanides are produced by addition of the oxides or
hydroxides to hydrocyanic acid. Those of the alkalies
and alkaline earths are white, soluble crystalline com-
pounds ; those of lead, mercury, and silver closely resemble
the chlorides. Mercuric cyanide, a sparingly soluble,
white crystalline salt, is formed by dissolving mercuric oxide
in hydrocyanic acid ; lead cyanide is also sparingly soluble,
and silver cyanide, produced by addition of potassium cyanide
to a soluble silver salt, is an insoluble white precipitate, un-
distinguishable from silver chloride in appearance. The
cyanide of silver or of mercury, when heated, yields cyanogen,
(CN)2, a colourless gas, possessing the characteristic cyanide
smell, somewhat resembling that of bitter almonds. Like
hydrocyanic acid, it is very poisonous. It burns with a
blue-purple flame ; it is fairly soluble in water and in
alcohol. It condenses to a liquid which boils at — 20°,
and freezes at — 34.4°. It is an endothermic substance,
being formed from its elements with an absorption of heat
of — 65,700 calories.
Isomeric Cyanides. — The formula of hydrocyanic
acid can be represented in two ways. It is possible to
conceive either the carbon or the nitrogen to be united
with the atom of hydrogen. In the former case, the
structural formula is H— C^N ; in the latter, H— N=C.
There is no method of determining which of the two
formulae is to be ascribed to the acid or to its simple salts ;
but salts with alcohol radicals are known to which one or
other formula can be ascribed. On distilling potassium
cyanide with potassium ethyl-sulphate, the following change
takes place :K-C=N + C2H5.KSO4 = CH3-CH2-C^N
4- K2SO4. Here it is known that the carbon of the ethyl
group is in direct union with the carbon of the cyanogen
for two reasons : first, when ethyl cyanide is exposed to
the action of nascent hydrogen (e.g. treated with tin and
1 84 MODERN CHEMISTRY
hydrochloric acid) hydrogen adds itself both to the carbon
and to the nitrogen of the cyanide group, and propyl-
amine, CH3— CH2— CH2— NH9, is formed ; and second, on
boiling ethyl cyanide with a solution of caustic potash
in alcohol, an acid with three carbon atoms, propionic
acid, is formed : CH3-CH2-C=N + KOH + H0O =
J*
CH3— CH2— C^ + NH3 ; dyad oxygen and monad
\OK
potassoxyl replace triad nitrogen. On the other hand, if
ethyl iodide, CHg— CH9— I, be boiled in alcoholic solution
with silver cyanide, the changeis : CH3— CH2— I + Ag— N=C
= CH3— CH2— N=C + Agl. Here the nitrogen is in direct
union with the carbon atom of the ethyl group ; this is known
because on boiling the compound with dilute acid, hydro-
lysis takes place, thus: CHg-CH2-N=C + 2H0O =
CH3-CH2-NH2 + H-CO-OH; the nitrogen remains
in union with the carbon. Hence it is concluded that
while potassium cyanide must contain K— C=N, along with
K— N=C (for both compounds are formed by the first
action), silver cyanide consists almost exclusively of
Ag— N=C. The name applied to the first compound,
CH3CH9CN, is ethyl cyanide, or, preferably, propionitrile,
seeing that it differs from propionic acid only in having
nitrogen in place of oxygen and hydroxyl ; while the latter
is turned ethyl isocyanide or ethyl carbamine, for it contains
carbon replacing hydrogen in ethylamine, CHgCH0— NH0.
Hydrocyanic acid may on the same grounds be termed
" formonitrile," for, on standing with dilute acid, it is
converted into formic acid by assumption of the elements
of water: H-C-N + 2H2O = H-CO-OH + NHg; and
cyanogen, for the same reasons, may be named " oxaloni-
C=N CO-OH
trile": | + 4H2O = \ + 2NH
C=N CO-OH
Ferro- and Ferricyanides* — Some of the double
cyanides are of importance, both commercially and from a
FERROCYANIDES 185
chemical standpoint. Among them is a substance which
has already been mentioned, potassium ferrocyanide,
K4Fe(CN)6. This compound forms large yellow
tabular crystals ; it contains ferrous iron, hence the
name ferro(us)cyanide. It is supposed to be derived
from a tricyanogen group, and to have the formula
K— C C— K
N >C— Fe— C^ N
I JN/ \N. |
K— C^ ^C— K
+
The ions of this salt are five, viz., 4K, and the complex
group Fe(CN)6; the fall in freezing-point of an aqueous
solution caused by the presence of a gram-molecule in
100,000 parts of water, and the conductivity of a similarly
dilute aqueous solution indubitably indicate the presence of
five ions. Moreover, the salt shows none of the reactions
characteristic of ions of dyad iron, such as precipitation as
sulphide on addition of ammonium sulphide, precipitation as
hydroxide on addition of alkalies, &c. The acid corre-
sponding to this salt, hydroferrocyanic acid, H4Fe(CN)6,
can be prepared by adding to a boiled solution of potassium
ferrocyanide concentrated hydrochloric acid and a little
ether ; it precipitates in white crystals. The zinc salt
and the copper-potassium salt, K2CuFe(CN)6, are in-
soluble ; the former is white, and the latter a slimy brown-
red precipitate.
On passing a current of chlorine through a solution of
potassium ferrocyanide, or on submitting it to the action of
any oxidising agent, potassium ferri(c) cyanide is formed :
2K4Fe(CN)6 . Aq + C12 = KC1 . Aq + 2K8Fe(CN)6 . Aq.
The new compound contains ferric iron, hence its name ;
the " c " is omitted for the sake of euphony. This
salt crystallises in dark red prisms, and dissolves in water
with an orange colour. The acid, prepared from the lead
1 86 MODERN CHEMISTRY
salt, which is sparingly soluble, by the action of dilute
sulphuric acid, and evaporation to crystallising point,
forms brownish needles. Here again the complex group
- Fe(CN)6 is one of the complex ions in solution along
+
with 3K ; and it is to be noticed that it now carries only
three electrons instead of four, as in the ferrocyanide.
Similar instances are to be remarked in elements of two
valencies ; and in the manganates and the permanganates,
the former of which have the dyad ion MnO4, while with
the latter it is monad, MnO4. The iron salts of ferro-
and ferricyanic acids are especially interesting, and some of
them are of commercial importance. On adding a solution
of ferrocyanide of potassium, boiled so as to expel dissolved
oxygen, to a solution of iron wire in sulphurous acid, which
is also free from dissolved oxygen, a white precipitate of
potassium ferrous ferrocyanide results : FeSO3.Aq +
K4Fe''(CN)6.Aq = K2Fe'Te"(CN)6 + K2SO3.Aq. If
these precautions to exclude oxygen are not taken, the
precipitate is light blue in colour, and is a common test for
ferrous iron. This compound is also formed when ferro-
cyanide of potassium is distilled with dilute sulphuric acid,
as in the preparation of prussic acid: 2K4Fe"(CN)6.Aq +
3H2S04.Aq = K2Fe"Fe"(CN)6 + 3K2SO4.Aq. When
boiled with dilute nitric acid, the white compound is con-
verted into a blue soluble compound, which may be
regarded either as potassium ferrous ferricyanide or
potassium ferric ferrocyanide, KFe"Fe'"(CN)6, or
KFe'"Fe"(CN)6. This same compound is produced also
by the addition of a ferric salt to potassium ferrocyanide :
K4Fe"(CN)G.Aq + Fe"'Cl3.Aq = KFe'"Fe"(CN)6.Aq
+ 3KCl.Aq ; or by adding a ferrous salt to potassium ferri-
cyanide : K3Fe///(CN)6.Aq + Fe"Cl,.Aq - aKCl.Aq
-I- KFe"Fe'"(CN)6. Aq. When mixed with excess
of a ferrous salt, it gives a blue precipitate named
CYANIDES 187
"Turnbull's blue": 2KFe"Fe'"(CN)6.Aq + Fe''SO4.Aq
3*sFc/'8Fe"/2(CN),05 and with excess of a ferric salt,
"Prussian "blue" "is formed: 3KFe"Fe'"(CN)rt.Aq +
Fe'"Cl3.Aq - FeyV"4(CN)ls + 3KCl.Aq. Potassium
ferricyanide, with ferric iron, gives a brown solution, which
may contain ferric cyanide. These colours are used as tests
for ferric or ferrous iron.
Chromicyanides, manganicyanides, cobalticyanides,
ruthenocyanides, and osmocyanides, are also known,
similar in formulae to the ferro- and ferri-cyanides. On the
other hand, nickel and platinum form double cyanides
similar in formula to K0Pt(CN)4. The platinum salts
are very beautiful, possessing the property of dichroism, i.e.
of transmitting light different in colour from that which the
crystals reflect ; moreover, only some of the facets of the
crystals have this property.
Silver cyanide is soluble in a solution of potassium
cyanide, also forming a double salt, of the formula
KAg(CN)2. Here the ions are K and Ag~(CN)2.
This salt finds two uses. First, it is the compound from
which silver is best deposited electrolytically in elec-
troplating (see p. 10). Potassium auricyanide,
KAu(CN)4.Aq, produced by the addition of auric
chloride, AuClg, to a solution of potassium cyanide, is
employed in gold-plating. Second, the existence of the
soluble ion, Ag(CN)9, furnishes a means of estimating the
amount of hydrocyanic acid in a dilute solution such as is
used for medicinal purposes. A decinormal solution of
silver nitrate, that is, one containing one-tenth of the mole-
cular weight of the salt taken in grams, or 17 grams per
litre, will react with 13.02 grams of potassium cyanide,
or with 5.4 grams of hydrocyanic acid, forming the
double salt, thus: AgNO3.Aq + 2KCN.Aq = KNO3.Aq
170 J30.2
+ KAg(CN)2.Aq. The solution of silver nitrate is added
from a measuring-tube or burette until a faint trace of
i88 MODERN CHEMISTRY
turbidity begins to appear ; this signifies that the reaction
given above has completed itself, and that the second
reaction— KAg(CN)2.Aq + AgNO3.Aq=KNO3.Aq +
2AgCN — has just begun. Every cubic centimeter, there-
fore, of silver nitrate added corresponds to the presence of
0.0054 grams of hydrocyanic acid in solution.
Metallic gold is soluble in a dilute solution of potassium
or sodium cyanide, the complex group Au(CN)3 being
formed, thus : —
4KCN. Aq + Au = KAu(CN)4. Aq + K.
The action of the potassium on the water is to liberate
hydrogen. But this hydrogen attacks the oxygen dissolved
in the water, and is removed by water. The process is
largely used in recovering gold from poor gold ores, or
from the " slimes," or mud left after removing the bulk of
the gold from the crushed ore, by amalgamating it with
mercury.
The addition of a solution of potassium cyanide to a
solution of a cupric salt in ammonia, which, it will be
remembered, contains the blue ions of the cupramine,
+ +
Cu(NH3)2, decolorises the solution. This is due to the
formation of the double salt, potassium cupricyanide,
+
K2Cu(CN)4.Aq, the ions of which are colourless. The
+ +
copper is not present in the form of cupric ions, Cu, hence
it does not give the reactions characteristic of these ions.
For example, it yields no precipitate with sulphuretted
hydrogen ; and this affords a means of separating copper
from cadmium, which is unaffected by addition of potas-
sium cyanide.
CHAPTER X
Borides and Carbides — Alloys: their classifi-
cation— The commoner Alloys.
Borides and Carbides. — These compounds have
been incidentally mentioned on p. 30 ; they have been in-
vestigated almost exclusively by Moissan and his pupils.
Borides. — Calcium, strontium, and barium borides
have been prepared by heating in an electric furnace a
mixture of borate of the metal with aluminium filings and
carbon. At the high temperature of the electric arc the
carbon reduces the aluminium oxide and prevents its
formation. These compounds form hard, transparent
microscopic cubes, burning only when maintained at a
red heat in oxygen, and attacked with difficulty by the
halogens. Their formulae are curious ; they are analogous
to the very unstable hydrazoates, M'(N)3, being Ca(B3)2,
Sr(B3)2, and Ba(B3)0; and their existence would point to
/B
a supposititious compound of the formula H — B<^ || like
\B
/N
H— N< || .
\N
Ferric boride, produced by heating together boron and
wrought-iron in an electric furnace, consists of brilliant
yellowish-grey crystals, burning brilliantly when heated in
oxygen, and attacked by nitric acid. The corresponding
compounds of nickel and cobalt, prepared in the same
190 MODERN CHEMISTRY
manner, form brilliant prisms. The formulae are FeB,
NiB, and CoB.
Carbon boride, CB6, forms lustrous black crystals,
nearly as hard as diamond, on which facets can be cut by
its use ; it is produced by heating a mixture of amorphous
boron and sugar-charcoal in the electric furnace.
Carbides. — Lithium carbide, Li2C2, is a white
crystalline mass, produced by heating in the electric
furnace a mixture of carbon with lithium carbonate ; its
formation is expressed by the equation: Li9CO3 + 4C =
Li9C9 + 3CO. It is decomposed at a temperature not
much higher than that at which it is formed ; hence the
exposure to the high temperature of the electric furnace
should be only a short one. The analogous compounds of
sodium and potassium do not resist such a high tempera-
ture ; they must therefore be prepared by exposing the
metal for several weeks to the action of acetylene under
pressure. This process yields compounds of the formulas
NaHC9 and KHC2 ; when heated, they change, with
evolution of acetylene, into the carbides Na2C2 and K0C0.
Like lithium carbide, they are white crystalline substances,
and with water acetylene is evolved : Na2C2 + 2H2O =
Calcium carbide, CaC9, has attained great industrial
importance owing to its serving as the source of acetylene,
now largely used for illuminating purposes. It was made
in an impure state in 1892 by Travers by heating to-
gether a mixture of calcium chloride, carbon, and sodium ;
but it is best produced by Moissan's process in the electric
furnace, by heating a mixture of carbon and lime to the
very high temperature (about 3000°) obtained in that
manner. It forms blackish-grey, lustrous crystals, at once
attacked by water: CaC2 + 2HOH = C2H2 + Ca(OH)2.
Carbides of strontium and barium are ma Je in a similar
manner, and have properties analogous to those of the
calcium compound.
Other carbides prepared by Moissan in a crystalline
CARBIDES 191
state by means of the electric furnace are : CeC9, LaC9,
YC0, ThC.,, which yield with water a mixture of acetylene,
ethylene, methane, and hydrogen ; Al4Cg, which is decom-
posed by water, yielding pure methane ; MnyC, yielding
methane and hydrogen ; and U2C3, the products from which
are ethylene, methane, and hydrogen. By heating the oxide
of the respective metal with calcium carbide, the carbides
Cr3C,, Mo.2C, W2C, TiC, and SiC have also been
prepared. The last of these has become known com-
mercially under the name "carborundum." It forms
extremely hard, blackish-blue, hexagonal crystals ; when
pure it is colourless. It is prepared on a large scale by
heating together in the electric furnace a mixture of car-
bon (coke) and white sand. It is used for grinding and
polishing metals and glass.
Steel, as is well known, differs from iron by the presence
of a certain amount of carbon, which induces the iron,
when cold, to persist in its allotropic state. This appears
to be due to a carbide of iron mixed with the excess of
iron in the steel. The compound has been found as a
meteoric mass ; it has been named cohenite, and has the
formula Fe3C. On treating steel with dilute acetic acid,
the same substance remains as a black powder. Its for-
mula is similar to that of manganese carbide, MngC.
Sllicides. — Some silicides have also been prepared by
aid of the electric furnace by heating elements with silicon.
Among these are Fe9Si, lustrous prisms ; Cr.7Si, Ni2Si,
Co2Si, Mn9Si, Cu0Si, and Pt2Si, with similar properties.
Magnesium silicide, Mg9Si, prepared by heating a mixture
of powdered silica and magnesium dust to redness, is
attacked by dilute acid, evolving a mixture of hydrogen
and hydrogen silicide (see p. 38).
Alloys. — The word "alloy" was originally applied to
mixtures of gold and silver with other metals ; it now
signifies any mixture or compounds of metals with each
other; alloys of mercury are, however, termed "amal-
gams." When two metals are melted together, they
192 MODERN CHEMISTRY
always mix, more or less. Some may be mixed in any
desired proportion, such as lead and tin ; others are par-
tially soluble in each other ; zinc, for example, dissolves
1.6 per cent, of lead, and lead 1.2 per cent, of zinc ; but
. on stirring up the metals together, there is always a layer
at the top of the lighter alloy of zinc with lead, and below
it the heavier alloy, consisting chiefly of lead. By raising
the temperature the mutual solubility of the metals in-
creases, and at a sufficiently high temperature it is probable
that they would mix completely.
Classification. — Alloys in general may be classified
under two heads : ( I ) definite compounds, in which the
elements are present in atomic proportions; and (2) mix-
tures in which combination has not taken place. To these
classes may be added a third — mixtures of definite com-
pounds with one or other of the components of the alloy.
As such mixtures are usually homogeneous, it is often a
matter of great difficulty to identify the definite compounds.
In many cases, too, it would appear that one of the metals
in the alloy is present in an allotropic form ; for example,
on treatment of one of the alloys of rhodium and zinc with
dilute hydrochloric acid, after solution of the zinc, the
rhodium is left in an allotropic form.
The constitution of alloys can be deciphered by several
processes. One depends on measurement of the electro-
motive force of a battery consisting of the alloy and a plate
of some resistant metal — for instance, platinum — compared
with that of a similar cell made with one of the con-
stituents of the alloy. To take a concrete example.
Suppose a cell were constructed of a plate of copper and
a plate of platinum dipping in some appropriate liquid, a
certain electromotive force would result. Imagine a plate
of tin riveted to the face of the copper plate, the electro-
motive force would now be that of the more electropositive
metal, tin. If a plate of bronze be substituted, supposing
it to contain free tin not in chemical combination with
the copper, then the electromotive force will still be that of
ALLOYS 193
tin against platinum. A chemical compound of tin and
copper, however, would have a less electromotive force
than free tin ; and as the tin in the alloy mentioned is
dissolved away, the electromotive force will suddenly fall
when the excess of tin has been dissolved, until it is equal
to that of the chemical compound against platinum. An
analysis of the alloy at this stage will reveal the com-
position of the compound. In this way the existence of
a compound of the formula Cu3Sn was detected.
The second method of determining whether an alloy
contains a definite compound is to compare the freezing-
points of various alloys of the metals. The presence of
a small amount of one metal in another in general lowers
the freezing-point ; and the freezing-point is continuously
lowered by successive additions, until the lowering reaches
a maximum. The mixture which has the lowest possible
melting-point is termed the " eutectic " alloy. The com-
position of this alloy does not necessarily coincide with that
of a definite compound ; indeed, metals which form no
compound with each other exhibit this phenomenon. If a
compound is formed, however, the melting-point rises to
a maximum on continuous addition of the second metal,
and that compound which has the highest melting-point
corresponds with a definite formula. Further addition of
the second metal causes a lowering of the freezing-point
of the definite compound ; and this lowering increases on
addition of the second metal, until a second eutectic alloy
is formed, one consisting of a mixture of the compound
with excess of the second metal. Further addition of the
second metal now causes the melting-point to rise, it may
be to the melting-point of the second metal ; in that case
only one compound of the two metals is capable of
existence. It may happen, however, that, after rising to a
certain temperature, the temperature again falls on addition
of more of the second metal ; in that case, the highest
temperature reached corresponds to the existence of a
second compound ; a similar change may even denote the
VOL. II. N
194 MODERN CHEMISTRY
existence of a third. By such means it is possible to de-
tect the existence of definite compounds between any two
metals. With ternary alloys, i.e. with alloys containing
three metals, although the phenomenon is more compli-
cated, this method has led to the discovery of several
definite compounds.
While alloys have generally been prepared by melting
the metals together, or by melting one of them and adding
the other, some alloys have been produced by submitting
mixtures of the metallic powders to enormous pressure.
"Fusible Alloys." — Among the eutectic alloys, some
are known as "fusible alloys." "Wood's alloy" con-
sists of two parts of tin, two of lead, seven or eight of
bismuth, and one or two of cadmium; it melts at 66°-7i°;
an alloy melting at 60° (Lipowitz's) consists of tin four
parts, lead eight parts, bismuth fifteen parts, and cadmium
three parts.
Among the few alloys of definite composition are:
ZnPt, Zn3Hg, Cd2Tl, Al8Mn, Sn4Pt, Cu3Sn, and PtHg2.
Attempts have been made to separate the constituents of
alloys by passing a current of high potential through the
melted alloy, with the expectation that electrolysis would
take place ; but no sign of such separation could be detected ;
the alloy conducts as a whole.
The following alloys, among others, find practical use: —
Sodium amalgam, made by adding small pieces of
sodium to mercury, warmed under a layer of heavy paraffin
oil; it is liquid when it contains under 1.5 per cent, of
sodium, and solid if it contains more. It is used as a
source of nascent hydrogen, for it is slowly attacked by
water, and more rapidly by dilute acids. On adding this
alloy to a concentrated solution of ammonium chloride, a
very remarkable phenomenon takes place ; the amalgam
swells up enormously while retaining its metallic appear-
ance ; the product is soft and of buttery consistency ; it
may consist of ammonium amalgam, and may contain the
complex group NH4, or, more probably, (NH4)2. On
ALLOYS 195
standing, it rapidly decomposes into mercury, ammonia,
and hydrogen.
The addition of a little magnesium to nickel lowers
its melting-point considerably, and renders it ductile and
malleable. A similar addition of a little aluminium to
iron also improves the qualities of the iron. The product
is called " Mitis steel."
" Galvanised iron " is produced by passing clean sheets
of iron through molten zinc. Alloying takes place on the
surface of the iron. Such plates, in a corrugated form,
are largely used for roofing buildings. As zinc is a more
electropositive metal than iron, the iron is thereby pro-
tected from rusting. Iron is similarly coated with tin;
but in this case, the iron, if exposed, is prone to rust, for
iron is more electropositive than tin, and is attacked by
carbonic acid in water more readily than the tin. Indeed,
rusting proceeds in an accelerated rate owing to the presence
of the tin, for the two metals form a couple.
To deprive commercial lead of the silver which it
almost always contains, zinc is stirred up with the molten
metal ; the zinc dissolves the silver and floats to the
surface of the lead ; it is allowed to harden, and the cake
is then removed. The silver and zinc are separated by
distillation of the more volatile zinc ; the lead is freed from
zinc by melting it in an oxidising atmosphere, when the
more easily oxidisable zinc is first oxidised, and can be
removed as dross. This is Parke's process for desilverising
lead.
The alloy of zinc with copper is termed brass, pinch-
beck, Muntz metal, and tombac. English brass usually
contains 70 per cent, of copper and 30 of zinc. It is
made by melting the copper and adding the molten zinc.
The addition of nickel (Cu 52 per cent., Zn 23 per cent.,
Ni 1 5 per cent. ) yields " German silver," of which spoons,
forks, and coins are made. Electroplate has usually a basis
of this alloy, and is covered with silver by depositing it
from its double cyanide with potassium. Zinc coated over
196 MODERN CHEMISTRY
with a superficial layer of zinc amalgam is not attacked by
dilute sulphuric acid, and is therefore used as the negative
pole of most batteries ; it is only on connecting with some
less electropositive metal that hydrogen is evolved from the
latter, while the zinc dissolves.
" Aluminium bronze " is an alloy of aluminium with
copper, containing from 2 to 1 1 per cent, of the former
metal. It resembles gold in colour, and is employed in the
manufacture of imitation jewellery.
" Ferro chrome " and " ferromanganese " are produced
by simultaneous reduction of ores of iron and chromium, or
of iron and manganese. Their addition in small quantity to
iron improves its quality. Iron containing about i o per cent,
of manganese is known as " spiegel iron," for it crystallises
in large brilliant plates. Tungsten, too, is sometimes added
to iron to increase its hardness.
" Pewter" is an alloy of 80 per cent, of lead with 20 per
cent, of tin ; plumbers' solder consists of two parts of lead
to one of tin ; " Britannia metal " is made of equal parts
of brass, tin, antimony, and bismuth.
" Bronze " is one of the most ancient alloys, and used to
be made by reducing together copper ores and tin ores. It
often contains twenty-two parts of tin and seventy-eight
parts of copper. Its hardness is greatly increased by the
presence of a trace of phosphorus. " Speculum metal,"
for astronomical mirrors, is made by alloying thirty-two
parts of tin with sixty-seven of copper and one of arsenic.
It takes a very high polish. Copper is easily tinned by
melting the tin in the vessel, and pouring out the excess ;
this is frequently done to vessels required for cooking.
"Type-metal" is an alloy of lead and antimony, con-
taining 1 8 per cent of the latter. It expands slightly on
solidifying, and consequently when cast in the mould it takes
an accurate impression and forms a clean-cut type.
The "Pattinson's process" is a rival of the Parke's
process in desilverising crude lead. The lead is melted and
allowed to solidify partially ; the solid portion consists of
ALLOYS 197
nearly pure lead. The liquid portion contains the silver.
By repetition of the process, the lead may be nearly com-
pletely deprived of silver ; and an alloy rich in silver may
be obtained, from which the lead may be removed by
cupellation.
Osmiridium is a native alloy of osmium and iridium ; it
is extremely hard, and it is used for pointing gold pens and
for the bearings of small wheels. An alloy of platinum
with 10 per cent, of iridium is the metal employed for
crucibles.
An alloy of copper and silver is used for coinage ;
English coins contain 7.5 per cent, of copper. The alloy
must be rapidly cooled, else it ceases to be homogeneous.
Gold is also alloyed with copper for coinage ; pure gold is
a soft metal. The English standard is eleven parts of gold
to one of copper ; in France and the United States, nine of
gold to one of copper. The richness of such an alloy is
measured in "carats." Pure gold is " 24-carat gold;"
" i8-carat gold" contains eighteen parts of gold and six of
copper.
The study of the chemistry of metallic alloys was for
long neglected, but of recent years much has been done.
It is curious to think that the successful solution of many
chemical problems is to be expected from careful examina-
tion of this class of substances, which was the first to engage
the attention of the chemists of the remote past.
INDEX
ACETIC acid, 112
Acetylene, 32, 36
Acid chlorides, 123
Acids, 40, 69
,, rneta, 124
,, ortho, 123
,, pyro, 125
Air, 2
,, analysis of, 4
,, liquid, 26
Alcohols, 67, 87, 88
Aldehydes, 88
Alloys, 191
,, fusible, 194
Aluminates, 78
Aluminium, 9, 16
,, hydroxide, 77
Amalgams, 194
Amines, 89, 171
,, salts of, 177
Ammonia, 32, 36, 37, 42
Ammonium halides, 65, 66
,, hydroxide, 86
Antimonides, 181
Antimony, 16
,, hydride, 38
Argentamines, 180
Argentocyanides, 178
Argon, 5
Arsenates, 129
Arsenic, 16, 17
, , hydride, 38
Arsenides, 181
Arsenites, 136
Atmosphere, 2
Auramines, 180
199
Auric chloride, 56
Aurocyanides, 187
BARIUM, 8, 16
,, dioxide, 92
,, oxide, 77
Basic oxides, 76
,, salts, 104
Basicity, 125
Bauxite, 6
Benzene, 49
Beryllium, 8, 16
Bismuth, 16
,, hydroxide, 77
Biuret, 175
Bleaching-powder, 141
Borates, 105
Borides, 189
Boron, 16, 17
,, fluoride, 59
,, hydride, 38
Brass, 195
Brin's process, 13
Bromates, 144
Bromine, u, 15, 23
,, hydride, 34, 41
Bronze, 196
CADMIUM, 17
,, hydroxide, 77
Caesium, 8
,, hydroxide, 74
Calcium, 8, 16
,, oxide, 76
Carbamates, 173
INDEX
Carbamine, 174
Carbides, 36, 190
Carbon, acids of, 112
,, chloride, 58
,, gas-, 12
,, hydrides, 32
,, monoxide, 93
,, oxysulphide, in
Carbonates, 106
,, acid, 109
,, basic, 108
,, native, 108
Carborundum, 191
Cerium, 16
Chemical action, rate of, 34
Chlorates, 142
Chloride of lime, 141
Chlorine, n, 15, 21
,, hydride, 34, 41
,, monoxide, 141
,, peroxide, 143
Chlorochromates, 150
Chlorosulphonic acid, 150
Chromamines, 178
Chromates, 151
Chromic acid, 152
Chromicyanides, 187
Chromium halides, 63
,, hydroxides, 77, 78
Chromyl chloride, 150
Classification, 30
Coal-gas, 12
Cobalt, 16, 18
,, amines, 178
,, halides, 63
,, hydroxide, 78
Coinage, 197
Complex groups, oxides of, 86
Complexity, molecular, 45, 99
Compounds, i, 30
Copper, 9, 16, 18, 23
,, hydroxide, 77, 78
Cupramines, 179
Cupric iodide, 56
Cupricyanides, 188
Cuprous chloride, 54
Cyanides, 182, 183
DISPLACEMENT, 15
ELECTRIC potential, 23
Electrolysis, 7
Elements, 6
,, properties of, 26, 27,
28, 29
,, separation of, 7
Ethers, 90
Ethylene, 36
FERRICYANIDES, 185
Ferrochrome, 196
Ferrocyanides, 185
Ferromanganese, 196
Fluorine, n
,, hydride, 33, 40
Flux, 16
Formic acid, 112
GALLIUM, 9
,, hydroxide, 77
Galvanised iron, 195
Gases, solution of, 3
German silver, 195
Germanium, 16, 18
,, hydroxide, 77
Gold, 10, 12, 23
Graham's Law, 3
Guanidine, 172
HALIDES, 50
,, double, 54
Halogens, hydrides of, 33
Helium, 4
Hydrazine, 42, 86
Hydrazoates, 170
Hydrazoic acid, 41
Hydrides, 30
Hydrocarbons, 45
Hydrocyanic acid, 182
Hydrogen, 12, 24
bromide, 34, 35, 37
chloride, 34, 35, 37
,, dioxide, 92
,, fluoride, 33, 35
iodide, 3^ 37
,, liquid, 6
Hydrolysis, 51, 53
INDEX
Hydrosulphides, 69, 81, 82, 83
Hydrosulphites, 166
Hydroxides, 69, 71, 72, 73, 74
,, properties of, 78
Hydroxylamine, 87
Hypobromites, 144-
Hypochlorites, 140
Hypoiodites, 134
Hyponitrites, 138
Hypophosphates, 137
Hypophosphites, 136
INACTIVE gases, 4
Indicators, 75
Indium, 16, 17
,, hydroxide, 77
"Insoluble" substances, 70
lodates, 134
Iodine, n, 15, 23
,, halides of, 60
,, hydride, 34
lodometry, 165
lodonium compounds, 91
Ions, colour of, 64
Iridium, 12
Iron, 16, 18, 19
,, halides, 63
,, hydride, 31
,, hydroxides, 78
KRYPTON, 5
LANTHANUM, 16
,, hydroxide, 77
Lead, 16, 18, 25
,, chlorides, 62
,, hydroxide, 77
Lithium, 8, n
,, hydride, 31
,, hydroxide, 74
MAGNESIUM, 8, 16
,, hydroxide, 77
Manganates, 152
Manganese, 18
,, dioxide, 102
,, halides, 63
hydroxides, 78
Manganicyanides, 187
Marsh's test, 39
Mass-action, 14, 62
Mercuramines, 180
Mercuric iodide, 56
Mercury, 12, 22
Meteoric iron, 31
Methane, 32, 36
Mixtures, I
Molybdates, 154
Molybdenum halides, 63
NASCENT state, 39, 42
Neon, 5
Neutral oxides, 98
Neutralisation, 75
Nickel, 10, 16, 18
,, halides, 63
,, hydride, 31
,, hydroxide, 78, 79
Niobium, 16
Nitrates, 127
Nitric acid, action on metals, 95
,, ,, oxidation with, 97
,, oxide, 97
,, peroxide, 98
Nitrides, 37, 169
Nitrites, 134
Nitrogen, 22
,, iodide, 59
,, oxygen compounds of,
132
,, chloride, 58
Nitrous oxide, 95
OSMIRIDIUM, 197
Osmium, 12
Osmocyanides, 187
Osmosis, 116
Oxalic acid, 113
Oxidation, 64
Oxides, 69
,, formation of, 80, 81
Oxygen, 13, 15
Ozone, 23
PALLADIUM, 12
,, halides, 64
INDEX
Palladium hydride, 31
Parke's process, 195
Pattison's process, 196
Perborates, 168
Percarbonates, 168
Perchlorates, 142
Perchromic acid, 167
Periodates, 134
Permanganates, 152
Peroxides, 192
Persulphates, 168
Pewter, 196
Phosphamides, 176
Phosphates, 126
Phosphides, 37, 181
Phosphines, 89
Phosphonium halides, 65, 66
Phosphorus, 18
,, acids, 133
,, halides, 64
,, hydride, 36, 37
Platinum, 12
,, halides, 64
,, hydride, 31
Polymerisation, 48
Potassium, 8, 17
,, hydroxide, 74
,, tetroxide, 92
Potential, electric, 23
REDUCTION, 25, 64
Rhodium, 12
Rubidium, 8, 17
,, hydroxide, 74
Ruthenium, 12
Ruthenocyanides, 187
SCANDIUM, 16, 25
,, hydroxide, 77
Selenates, 159
Selenic acids, 159
Selenides, 69
Selenious acid, 146, 149
Selenium, 12, 22
,, halides, 60
,, hydride, 39
Silicates, 115
Silicides, 191
Silicon, 16, 17
,, fluoride, 59
Silver, ID, 12, 16, 23
,, hydroxide, 77, 78
,, oxide, 79
Sodium, 8, 17, 18
,, dioxide, 92
,, hydride, 31
,, hydroxide, 74
Solder, 196
Solubility-product, 83, 84, 85
Solution of gases, 3
Spinels, 100
Steel, 21
Strontium, 8, 16
„ oxide, 76
Sulphamides, 177
Sulphates, 159
Sulphides, 69, 81
Sulphites, 147, 148
Sulphonates, 148
Sulphur, 12, 22
,, ethers, 90
,, halides, 60
hydride, 33, 39
,, trioxide, 155
Sulphuric acid, 155
Sulphurous acid, 146, 149
Sulphuryl chloride, 149
TANTALUM, 16
Tellurates, 159
Telluric acid, 159
Tellurides, 69
Tellurium, 12, 22
,, halides, 60
,, hydride, 39
Tellurous acid, 149
Thallium, 16, 18
,, hydroxide, 77
Thermal data, 145
Thio-acids, 130
Thiocarbonates, 106, no
Thionates, 166
Thiosulphates, 164
Thorium, 16
,, hydroxide, 77
Tin, 16, 18, 24,
,, chlorides, 62
INDEX
203
Tin, hydroxides, 77
Tinned iron, 195
Titanium, 16, 17
,, hydroxide, 77
Tungstates, 154
Tungsten halides, 63
Type-metal, 196
URANATES, 154
Uranium halides, 63
Urea, 174
VALENCY, 61
Vanadates, 129
Vanadium, 16
WATER, 32, 35, 36
,, of crystallisation, 53
XENON, 5
YTTERBIUM, 16
Yttrium, 17
,, hydroxide, 77
ZINC, 17, 19
,, hydroxide, 77
Zincates, 78
Zirconium hydroxide, 77
THE END
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